My Calibration Cube/Die combo. It is 20mm in each direction. There are other Dice Calibration Cubes. One in particular is vary similar to this, but the X is on the opposite side and I didn't like that the number dots are not in the standard position. There is a square sided version and a chamfered version.
When you are finished calibrating, you can use these as regular dice. Or give them away as gifts.
Not a remix, as I drew this from scratch in Fusion 360.
Update Jan 22, 2019:
minor fix to the #2 hole positions and #1 hole