This is a little adapter to store a loose Nintendo DS cartridge in the GBA accessory slot of a DS Game Case. Also fits 3DS cartridges.
As you can see in the picture, the reason behind this is I have a copy of Metroid Pinball without the case and a copy of Metroid Prime Hunters, with the case. I figured this little adapter would give Pinball a nice home on my shelf. Since other people might have a loose game or two with a case this should make a suitable adoptive home.
Because of the window on the front, I recommend printing this vertically with a raft. This makes it easier to remove the support material.