I'd use Peter's second method, finishing off with a simple-to-make trammel, router and straight cutter. A trammel is just a piece of ply, MDF, etc. with holes cut for the router and a suitable hole at the correct distance to insert a pivot nail into the underside, like this (click to enlarge):
trammel.jpg
You just need a compass (or tin lids, a ruler, a drill and a jig saw to make one.
Ray