Let's say you have a loved one sent to a prison, and you need to smuggle them a 3DS inside a Game Boy. (Look, just go with it). Goteking, who gave us the iPod-in-a-Dreamcast-VMU, now supplies a 3DS case made from a Game Boy.
It's like the turducken of gaming handhelds. Goteking supplies a fake monochrome screen and cartridge to complete the look. He even built a case out of a clear Game Boy, which kind of ruins the surprise but hey, it's still pretty cool.