I love my GameBoy and still use it today, even though I got it a long time ago. Some fun accessories wouldn't be amiss, now would they? How about a little personalized case, and because I like 3D printing so much, why not prep it for that?
Step 1: Base of the Box
I'll assume you know how to use TinkerCAD's website to set up an account, so I'll skip straight to the fun stuff. First, drop a cube into your workplane (That big blue grid). Then, drag the tiny white cube at the top of the big red one down, until it's about 5 millimeters thick. Then, drag each of the little cubes on the sides in turn so that they each are 100 millimeters long. Use the corner one if you're a pro. You want something like this.