A string will stretch, wire? A long board or two screwed together with pins or some such as points at the ends.
I have used another technique, used by boat builders, where you use a much shorter board with 4 pins, one of which is a pencil, to mark the curve, but for the life of me I can't remember how it works. And a quick google search yields this...... I didn't have is exactly right but this is the method I used.
http://books.google.com/books?id=V8L8y1J...p;q&f=false 
