Deep Dive into Product Development - MARTL Sacoche Bag

A year ago I published a deep dive into the design development of the MARTL, which I called Undercover Sacoche at that time. The product was not finished yet, but now it is, so I thought it's time to add the final stage.

I want to walk you through the entire design process of the MARTL. A super-slim sacoche, but opens like a fan. It is my most experimental design, a form built around a specific daily minimum.

We‘ll dive deep into asking the right questions, solving problems, technical development, and—most importantly—decising-making. That includes balancing intuition and straightforward logic.

Stage 1

Inspiration for this sacoche bag came from two directions:

1. There‘s still no elegant way (speaking of dudes like me) to wear your daily essentials low-key.

My most essential item is my phone. I have a Max Pro Super Big One I don‘t like carrying in my pants pocket; I still do sometimes out of convenience, but it‘s just too big. In response, my wallet has naturally shrunk over the years, yet my ID, driver’s license, and the bonus card for my trusted burger joint are still essentials. And then there are keys—those bulky buggers. Why are car keys getting bigger and bigger? And what I can‘t live without: music. I don‘t leave the house without at least a small pair of earphones.

The mass market adapted to the Zeitgeist by selling bags like this:

I like them, I wore them a lot myself—but sometimes they just look stupid. And I don’t know, opening a zipper just beneath your chin looks and feels awkward.

2. Once in London, I found this bag:

I’m still intrigued by the closure. It‘s not particularly practical (especially when the belt runs right through the middle like this), but using it is a delight. It reminds me of the feeling you get when snap-closing the case of Apple’s AirPods—just satisfying. Naturally, I wanted to adapt this principle somehow, somewhere.

So, what do I want this bag to be?

• Carry a (big) smartphone
• A small wallet
• Keys
• Something like AirPods
• Keeping those save
• The ability to grab my phone as quickly as if it were in my trousers
• A slim profile so it can sit under a jacket
• No tactical aesthetic—something simple, elegant, and delightful to use

That’s how I’d describe it now. But weeks ago, when I started designing this sacoche, I also wanted room for a small notebook and pen. I even imagined it fitting a larger wallet. I thought: people are different, and the bag should offer flexibility. But inspiration rarely comes as a single idea—you have to start building and see where exploration and testing take you.

Stage 2

Rough Prototyping

I began the project by laying out my essentials on the table and wrapping fabric around them. Usually, I start with quick sketches—just doodles to explore shape and function. But this time, my gut told me to jump into physical prototyping. I needed to play with real objects, tetris them into the most compact and accessible form possible.

That process led to some happy accidents. For example, I discovered that folding the pockets instead of sewing them onto the lining creates these smooth transitions:

When you put your hand in or pull out some weirdly shaped object (looking at you, keys), there’s no resistance. Nothing awkward. It seems like a small thing, but after using the later prototypes for a while, it just feels smooth. Bonus: this method also helps during production. I only need one fabric pattern, and the z-fold lets me add cushioning from the other side later on.

The first prototypes also defined the central theme: The bag opens like a fan. Your essentials are presented in an orderly, almost ceremonial way.

And how does it close? Gravity. Just like the London bag—it closes itself.

These are the best moments in design, when inspiration falls into place and just feels right. Even if you know that there are hurdles ahead and it’ll be tricky to solve certain details, it screams: You‘ve got something here!

One of the tricky bits: The fan needed to both look good and fold inwards cleanly. I also wanted to hide as many seams as possible, so my first attempt—just sewing onto the edges—didn’t work.

I had to sew them into the main seam to give them direction and connect the sides after inverting the whole bag. Can you follow? I could go on about how I stabilized the fan, but I guess you‘d fall asleep...

With the second prototype done, I felt confident enough to try proper materials.

This was the first time I worked with Italian wool. It represented the feel and quality I imagined for Geist. Like most of my advanced prototypes, the outer material is upholstery fabric—robust, cozy, tactile.

The idea to use the same belt for closure and carry doesn‘t work. Not because of access or security, but because opening it shortens the belt, which makes the whole bag ride up toward your chin. Reaching for your phone feels (and probably looks) really awkward.

Also, the size and shape were off—it just looked too big. And a little… boring. My jack-of-all-trades approach and my self-proclaimed technical ingenuity messed with the proportions and the original idea. Honestly, I felt stupid wearing it. So under the vest it went—not for subtlety and style, but out of shame.

As a result I left the country and questioned my reason for existence:

Sometimes you need to let your subconscious do the work. It’s like training: recovery is where the magic happens. I thought a month of downtime would mean I’d forgotten everything I learned in the past 6 months. But the opposite was true—my mind felt clearer, my steps more intentional, creative work flows more naturally.

Stage 3

Three main things needed to change:

1. Solve the belt issue without loosing the unique closure and fan movement.
2. Make it way smaller—just fit the essentials I mentioned in the beginning.
3. Try thicker fabrics to see if padding could be eliminated.

Here’s the fourth version I came up with:

Now there are two belts. One to carry—hooked to the top loops (now no longer metal, for weight and simplicity). The second belt runs through the loops and down the back, forming a hanging handle.

That way, you open the bag like before—but to close it, you pull down the handle, and magnets are supposed to do the rest (not in this one yet ;)

At first, I thought: is this too complex for such a small bag? Turns out, it’s actually more convenient—and faster—than a zipper on a sling.

And you might think: okay, but what about theft, or rain, or whatever. Fair. But the user experience is really satisfying and unique. Design is to make it work and to test thoroughly—in real life.

But… materials.

They looked wrong, and they were way too stiff. Folding them into these tricky little corners nearly made my fingers bleed. And the handle appears to have been excreted rather than created.

Additionally the exterior is still off, it‘s still boring and missing details. Also, the layout’s still off. A big phone sits too close to the top opening, so the magnets (which were not up to the task) are basically doing…

So: back to thinner materials for better precision, plus strategically placed padding (because: keys and phones). The layout needs some minor tweeks and I‘ll test wether a rope is a better handle. I think they‘ll fit the top of the bag better then the wide bands.

Stage 4

The Fifth Prototype

I was still not happy with the shape. This tends to happen when you skip proper silhouette sketching in the beginning of the design process—you end up paying for it later.

Still, there’s something about this bag that draws me in. E.g. I'm particularly proud of the transition from the exterior fabric to the interior lining:

Maybe the right material combination will make the simple shape shine. A more visible pattern perhaps that offers the level of detail needed.

Stage 5

Back to the drawing board

In the car industry we always talked about key sketches whenever a proposal captured the character of the product. This single or multiple key sketches stayed on the wall even if glossy renderings or polished models were at the center of attention. But whenever in doubt it functioned as a north star guiding you back on track.

My key sketch for the MARTL probably couldn't be simpler. Wings to house the sling strap and giving the bag a distinct shape which was lacking completely. The whole appearance doesn't shout conductors's bag anymore (how my partner lovingly called my first prototypes). A slight overlap of the lid makes the bag easier to open and shields the internals.

The Final Prototype

The other detail I wanted to achieve was to make the pulling cord run inside the bag. That obviously makes the bag more complicated to make, but looks way cleaner and is just more robust in the long term.

The final bag I sell on my website (link) is very close to the prototype in the images above. I sent a few of them to testers and after feedback only had to improve minor details.

The MARTL turned out to be for minimalist carry. It offers room for a smartphone, a slim wallet and keys. That's it, and it's intentional. You don't really need to carry around more. If you're on the edge, and not sure if that's enough for you, I'd recommend to go for the JIRO.

I'm really really happy about what the MARTL Sacoche became, and I'm super excited to see it in your hands and how it becomes part in your day to day life.

I imagine this bag working well under an unbuttoned jacket or shacket. Especially these days, when men around my generation like to avoid overt decoration, small, thoughtful details like this could add just the right amount of personality to an otherwise calm outfit.

Thanks for reading. See you around.