Summary

Inspired by the center finder published by DrPeper here:
http://www.instructables.com/id/Homemade-center-finder-tool-quick-cheap-easy

STL for 3D printing, since I do not have a laser cutter in reach.

You use a center finder by holding it against the perimeter of a round thing on some different points and drawing a line along the long side. The crossing of the lines should be the center of the circle.