heck you could use some of them large kids playing card's for the side art.
it maybe what they did with that and just cut it out with a razor after its glued or something or perhaps a calender cover or something

the shape for the side's would be easy becuase you could take a 1" make the shape you want and then rip it down the center on its side to get two exact matching side panels.
I never was a fan of the original GB myself but a gba sp version is temptin just to try it and see what happens in the end.
I also would not mind trying to find a way to hook up a cell phone screen to a gba for a way better picture beings the screens on the phone's nowaday's are so much better.