I came across this 10″ Round Center Finder on Amazon. $14 on 3/21/2020. I ordered one a few months back.
It works pretty good. It is fairly well made out of 1/8″ plexi with good printed lines that do not scratch off to easily.
I improved mine by highlighting the 8″, 6″ and 4″ circles with red, green, and blue sharpie permanent pen.
I use an Ice Pick in the center of mine rather than a nail.
I like it better than my home made plywood circles because it is transparent. It allows me to see where the pith is on a log and play around with putting the pith dead on center or not on center.
Yep-had one for years. I like the marker idea.