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.
Step 2: More Building
It's going to be boring for only a few more steps, I promise. Copy the big flat thing you have, and paste it. Now move the pasted block around until it is perfectly aligned with your first block, but right on top of it (Hint: You can move things up and down with the arrow above them.). Then, resize the pasted block so that it is exactly 36 millimeters high. It should look like this.
Step 3: Cutting!
Almost Exciting! Copy your newest block, then paste and align it, BUT NOT HIGHER THAN THE ONE YOU JUST COPIED! Then take 5 mm off of every side, and click the button that says "hole". That block you were just tweaking is now empty space! Now click and drag over your big block and it's hole block, selecting them. Now click "group" to group them together for convenience. It should look like this.
Step 4: Most difficult step ever!
You're not stupid, try to do this yourself! That cap has a hole the size of the rod in it, by the way.