Slate (Game Boy Advance SP Mod) Backpack

A simple bracket that clips on to the back of the screen on a Slate (modded Game Boy Advance SP) and allows you to clip in an additional Game Pak. A single bracket works fine but I found better results with two. Or you can just stretch the z-axis in your slicer.

Works with GBA and GB/C carts though with GB/C carts, you'll definitely want at least two brackets and, of course, the cart will stick out the top. 

Depending on your printer, you may need to scale this model up or down a percent or two. It worked fine for me as-is on my Ender 3 Pro but some beta testers reported having to scale.