With a watch battery, an LED, and a 3D printer.
Step 1: Do It.
See the pictures above to design your case. All you have to do is design one like this or you can make it different. We made a spaceship one with two holes for LED leads, an a normal one. For a button, I use my finger to push down the extra lead.
I made it to where the battery wouldn't fall out, also pictured above.