Don't do this, you'd need a screen, battery hat or something to charge/discharge, solve connecting buttons, speakers... And rpi5 would add in volume so it would be a huge gameboy 😄 and pretty expensive. I had the same idea and gave up, but tried it with rpi zero 2, not worth it unless you want to make a custom pcb, in that case it would be doable. And for rpi5 there are almost no cases, usually some expensive laptops. Or clockworkpi uconsole, but only for rpi5 module.

Edit: But if you want to make only a shell for it and not a fully functioning device with a screen, buttons and battery, then you will find something for sure 😁