There are many versions of this tool on Printables.com and elsewhere but I couldn't find one that worked for me. Thanks to all those creators but I felt a redesign was in order for my needs so here's my take on a center finder.  This one is sturdy and has a chamfered marking channel that works with the marking tools I have in my shop. 

One of the files included here also has holes that can act as center and radius for drawing circles.