I got my first "minimal wallet" in 2014.
For a few months I tried using cards and bills only. However, living in the Netherlands, coins are still quite frequently needed (parking, vending machines, public toilets, etc).
That's when I started thinking: "How can I carry some coins, while keeping my wallet as slim as possible?"
After trying a couple of things, I 3D printed the first version of Cavity Card. Only after a couple of friends and colleagues started asking where I bought it, I decided to start selling them too.