trammel points
 

www.madmasons.com/trammels.htm

lots of great feedback, most practical design. more info at the site. they are very inexspensive for now, and people love them. throw any sized radius with your own beam compass using whatever you have laying around.

edit: here's the ebay feedback on these:
http://feedback.ebay.com/ws/eBayISAPI.dll?ViewFeedback&userid=milehigh_woodcra fter&ssPageName=STRK:ME:UFS