Building a device inside a Game Boy shell is an easy way to utilise a pre existing design. It makes an easy starting point plus nostalgia!
I have uploaded the eagle board files for my GBC and GBP common ground boards, but for those not comfortable with exporting gerbers, uploading them, finding a board fab - I uploaded my pocket board to PCBWay so you can get them made from there in large quantities. All I ask for is credit, but go wild!
Board files via github: https://github.com/nataliethenerd/CommonGND
Here's a quick vid of me explaining these boards.