After starting my Nintendo collection, I decided I wanted a way to store all of my games on my DVD shelf, while keeping them nicely on display. With a bit of measuring and searching for inspiration on Thingiverse, I came up with a solution.
Each large cartridge rack (NES, SNES, Genesis, N64) holds six game cartridges. Each small cartridge rack (GB/GBA, DS/3DS) holds twelve game cartridges. Racks are the same depth as a standard DVD case, so they work well on DVD shelves or as bookends.
A small note on Genesis cartridges--I have noticed that for the handful of games I own, there's a noticeable discrepancy in exact cartridge measurements. I've sized the Genesis rack to hold my fattest game a little on the snug side, while the thinner carts are a little bit loose.
Updates: Added Game Gear and edited the N64 holder to be a better fit. Unfortunately, Game Gear games are much thicker than even GBA games, which means there can't be a full dozen in the holder like the other handhelds and still fit on DVD shelves. I've started remaking these in Onshape and will post links to models that I have completed on there. Added another GB model that has a raised rear slot so that rumble games like Pokemon Pinball can be placed there with the labels facing forwards. Virtual Boy added.
Game Gear - https://cad.onshape.com/documents/8864feda5e61a2b2dd493981/w/13af68de48373982ad0880c8/e/e12ef2f80fc95a4c72d6a529
N64 - https://cad.onshape.com/documents/b8020e7e857fc6ecd745f8f0/w/bdb76c491a949c98a85ecc3f/e/0c2b806d9880f32e9c079a18
GB/GBA - https://cad.onshape.com/documents/fc2e9ef2a7c4a9b55ac401fb/w/710a857017efc9e1ddba5b1a/e/4e23dde1a02d86e487a4aa9f
VB - https://cad.onshape.com/documents/5643350626e78a4f2f0a279e/w/114e577516022a8e1cb861e4/e/3c60379262079d03b0577390