I hope I can describe this clearly without a drawing. Here's what I do:
first draw a straight line 34" long. Make a mark 5" in from one end of the line. Now draw an arc, using the end of the line as the center point and with a 29" radius, so that the original straight line is in the middle of the arc. Then draw another arc, same radius, but using the 5" mark as the center point. You'll have two arcs that are not concentric, so that the belt will taper from the midpoint (which will end up at the center of your back) to the ends at the front. You can adjust the length of the belt easily to suit your needs. This pattern includes about a 1/4" seam allowance, and using 1/4" thick foam I cut the foam about 5/8" less than the belt (which means using radii that are 29 5/16" and 28 11/16", respectively, for the top and bottom of the belt, and using the same center points.
Hope that makes sense. It's really obvious on a drawing, but I don't know how I'd put a drawing into my post, other than having it as an image hosted elsewhere on the web, which I don't.