I just bought a official GBA case from Dollar Tree (for $1.00 of course), and its great. I ripped off the GBA logo and replaced it with my own sticker.
Here are some crappy pics made with my scanner:
The Case, with its semi-hard shell and form-fitting design for added protection agains drops and stuff.
The Case, showing how well the GP32 fits. The other side has a bag that can fit about 8 batteries, a couple SMC's and a pair of earbuds. On the back their is a strap for attaching it to things (like belt loops). It works well for that as its alot less clunky and big than the other case.
The old GBA logo plastic thingy.
My GP32 logo sticker. The original is a .svg, for unlimited scalability.