-My print wasn't great, if anybody else has better photos please let me know-
Designed to plug the gap where a cartridge would go. Allows you to remove the metal backplate inside of the gameboy case to allow more room for electronics.
I would recommend gluing in.
Added a version with a recess for a sticker.
I used Illustrator to lay out a 2D design and brought it into the third dimension with 123D Design.