Designed a customizable USD change holder for my car. You can choose its size, how many of which coin slots there are, how they are labeled, etc. Simple concept, I just wanted to be able to tweak it into exactly what I wanted.

You can use Thingiverse Customizer or OpenSCAD to modify the parameters. With a little bit of OpenSCAD know-how and effort, you can also add non-US coin types. If you do, feel free to release remixes or alternative versions.