I'm not sure if anyone has posted about these shells yet, but...
Rose Colored Gaming has some really nice DIY Gameboy shells, if you don't want to ruin a perfectly working Gameboy but still want to use a brand new case. Comes with replacement buttons and screws too, which is nice!
Not a bad price either, and great quality. I bought a Black GBA shell for my project and was surprised at how nice everything was, definitely recommend the glass screen protector too, really worth it!
P.S. I swear I'm not a sales associate
Hopefully this will help someone out there looking to start a project 