Sometimes you just want to carry one extra Nintendo Switch cartridge when out and about (physical versions of Xenoblade Chronicles 3 and Blaster Master Zero 3 for me at the moment). I couldn't find anything I really liked, so I whipped up this one in AutoDesk Inventor. It took a few iterations, but I think I nailed it on the 5th try. The opening is keyed, so the cartridge will only go in one way. It also lines up the retaining nub to keep the cartridge from slipping out.
The author hasn't provided the model origin yet.