I've seen a dozen or so of these designed throughout the web, but I wanted to try my hand at it and see if I could improve on a few things.

An 8x3 magnet goes in the bottom white portion of the box as well as one hidden in the “button”.

I printed this without any supports on any of the parts. I did, however, add a brim to all of my parts. If you have solid bed adhesion, do away with them. You'll have to print 2 black bands and stack/glue them.