Gameboy cartridge tray (or for whatever other rectangular things you want to organize). The https://openscad.org/ script has parameters for the number of slots, slot size, wall thickness etc… so you can tweak it to meet your needs. STL files from my test prints are also available.
Based on Gameboy cartridge holder by Liartes
https://www.printables.com/model/103541-gameboy-cartridge-holder
The author remixed this model.