Wanted a nice countdown die for playing magic, one that was really visible. Figured, might as well use the inside to house actual dice. Measures 3" between parallel faces, and uses 10 1/4" diameter, 1/16" deep magnets to close the box.
I liked the one at http://www.thingiverse.com/thing:105296, but I wanted arabic numerals in a countdown direction (countdown dice have consecutive numbers next to each other to help in tracking life points, rather than random number generation.)
Largehole files have a slightly larger counterbore to offset overextruding so the magnets fit better. BTW, I used a soldering iron to melt the magnets in -- don't do this -- the heat seems to kill the strength of the neodymium magnet.
d20closed_flat is just a large d20 countdown die.