I have created tons of Genesis, 32X and SMS slip cases and even one for the Sega CD Back-Up RAM Cart which came in an odd-sized paper box. I also have my Sonic & Knuckles in a real box now as well... with proper insert. I usually do all of my work at 400dpi, though. It's fun!
I am thinking that my uncle has a lot of those Disney-DVD white boxes that he wouldn't mind swapping out since he has kids. Not sure if they have their heart set on those boxes remaining white or not. I'll ask!