I'm not at work anymore to give you a drawing, however, the bottom diameter is 1/2" x 1/8" high transitioning to 3/8" dia. another 3/8" high for a total of 1/2" high. The inside diameter .12" x 3/8" deep, but it should have a harder plastic insert 1/8" diameter x 3/16"long inserted after to keep the speedo button from cutting or pushing thru the top of the rubber button. The reset plunger on my bike was not flat on the top but cut on an slight angle so the insert should be flat to push on it and not round.
I just glued it on because there's no space to get this thru the lens, and I didn't try taking the speedo apart.