Summary

This is a copy of a lock-down knob from a Craftsman router table. See photo, I copied the yellow part. It's designed to print with the threads already in it. Print it, test thread a piece of all-thread or a bolt into it, then take it out, put a dab of epoxy or superglue on the end, and thread it back in. Go all the way in if possible for strength.

Print from a tough plastic (ABS, or PETG, etc.), at least 4 walls, a thick roof and floor, and 30-40% infill.