Use a caliper to measure the front-to-back width of the thumb for the thin part of an oval hole. Use a regular round fitting to determine the side-to-side width. Skill is needed to figure at what angle to drill the oval, or measure an old ball with an oval insert to determine the correct angle to drill the oval hole. Using a drill jig that has an X-Y axis is best. Using trigonometric functions, you can figure how far and in what directions to lay out the two hole drill pattern.

As to why drill instead of an oval insert? Feel for some people. Shop inventory for others.
Top weight has to be higher in the ball. When you drill for inserts you use a big hole. Oversize thumbs like mine require a 1-3/8" hole almost 3" deep for an insert. When I drill it is only 1-1/8" by 1" oval  2-3/4" deep. If you want to keep top weight (and I do ) this requires a ball with high top weight which is not always easy to find or stock.