Okay...here's how I'd try it. Use some hardwood scraps to make a spine and two wedge-shaped chocks, then mount them to a small panel of plywood or masonite to create a rigid jig. Use a common quick-release squeeze clamp, Pony pipe clamp, or even a large C-clamp to apply gentle force where the arrows are pointing. Go slow and check your progress often. (A circle drawn on a large piece of cardboard provides an adequate standard.)
Depending on one's woodworking skill, the jig could be improved with curved, adjustable chocks and a matching wooden buck to protect the inner curve of the rim. But, what I've drawn here is probably enough to prove the concept.
View attachment 18437
If you build it, practice on a discarded rim FIRST! Just sayin'
