If you have a router, making a trammel for it is just about the only way to get a perfect circle.
http://www.woodworkingtips.com/etips/etip030201wb.html
I've tried to use a rotozip with a circle cutting attatchment in wood, and I didn't have very much luck. You can't control the depth of the cut, so you have to cut the entire thickness in one pass... the bit had a tendency to wobble in the collet, and the circle came off imperfect. I've made a trammel for my router, and cut the circle 1/4" deep at a time, until I got all the way through. It made a perfect, smooth circle.
Good luck.