Here is one way to do it without a radius dish. I made a batten with a 28 foot radius and one with a 15 foot radius and have used them for 8 guitars now. Very easy - takes a little more time to glue braces, but I really like John Arnold's idea of 2 of them with a sloppy "x" joint to support under both legs of the "x" brace being glued at one time - I just do one at a time now.
Start here and look at a half dozen on shaping and gluing braces, then jump ahead some to see the battens used for sanding the rim. I do the gneral brace shape before I glue them:
https://www.flickr.com/photos/ruby16...7688488198220/
Scroll left and right, text below, click to enlarge.
Also, someone gave the formula for the sagitta, and here is how you make a true arc easily and quickly:
https://www.flickr.com/photos/ruby16...7635172644427/
Ed