This remix of https://www.thingiverse.com/thing:1037301 adds two holes and matching pegs. One fits a standard hex bit mount, the other fits a 3.5mm headphone mount, giving some commonly available items to compare with. If properly calibrated, the two pieces will fit each other beg-to-hole and U-to-U.
The STL files come with three variations of tolerance added: 0, 0.1mm and 0.2mm. I didn't get the 0-tolerance pieces to fit until I changed the Perimeter setting in Slic3rPE to 65%.