Greg,
For the best results, you would have to get a custom made ring. If you get one either too big or too small, it will hurt immensely. Mine is made from moose antler, and it could not fit better. It has to slip past the thumb joint with a little force, and after that, it should not be too tight. I tried a few rings at a jewelry store until I found one that fit well. Then I had the inside diameter measured, and also measured the length of my thumb pad, from the first crease to the tip of the thumb.