If you're doing the kind of cool stuff we all want to do with a micro:bit in the classroom, you know how hard kids are on tech. And if you're trying to have them build cases for the micro:bit out of cardboard... well that's one broken button away from a $20 replacement. (Unless you can micro solder. Then you're among the cool people.)

Here's a scale accurate analog. All the important raised pieces are the same shape, size, and location as a real micro:bit. But most importantly, this one isn't going to be a horrible disaster if it breaks.

You won't even need supports. Just print it with the top down, and you'll be good to go.