This is to help you align your tap or drill to be perpendicular to the work surface. I have included the SCAD file for you to tinker with, or you can just print the default, which has 9 holes, from 2mm to 10mm in diameter in 1mm steps.

You will probably have to ream out the holes with an appropriately-sized drill bit to ensure smooth turning. I would also recommend printing at higher infill, since you want this thing to be heavy and strong (as much as plastic can be in any case).