My 2 cents worth, 1 cent a pop...
Option 1:
https://www.youtube.com/watch?v=gpHKv8UOYf8
Very handy tool, you could just make yours to size and use any cutting bit available.
Option 2:
https://www.youtube.com/watch?v=6dTzIHCPBuk
Option two is a must for anyone who wants an easy circle cutter imo.
Steve