Battery cover for my original GBA was broken for a while, so I decided to model up a replacement for it! Works great printed off of my Printrbot Simple 1405, but YMMV.
I've uploaded a second file that is the same thing, but with a snap fit that sits more flush to the back of the cover. This should let your GBA sit flat on its back without wobbling around.
If you'd prefer to purchase this from me, check out my Etsy or my Shapeways stores!
Made this while on break and no calipers on hand, so I had to make do with a normal ruler. Some of the measurements might be off because of that, but for the most part it works fine!
I printed this on its side (hope you can see the lines in the picture) at a 200 micron layer height. You can choose to print it flat if you want, but I don't think it will give you the same flexibility and might be prone to snapping at the bendy parts if you do.