Heck, I not only used to make 'em, I used to
sell 'em!
The local flea market had one vendor who sold the absolute perfect sized ball for a ballgag! What I didn't know the time was that they glowed in the dark -- great sales gimmick!
To make, I drilled a hole through the middle of the ball, using a this drill bit and then a larger one, then a larger one still. I made a narrow leather strap and pushed it through the hole, with enough on either side to rivet to a nickle-plated ring. On the other side of the rings went the straps, one with holes and one with a buckle -- or one side with a
small D-ring and a series of punched rectangular holes on the other so it'd accommodate a small padlock.
Using larger rings I made a variation with a chin strap that buckled, and an upside-down "Y" strap that went from the rings to a single strap that went over the head and Y'd out again to loop around the straps. The strap over the top of the head had a little D-ring as well.
Then my own invention: The harness strap rivets were replaced with snaps, so the straps could be removed and you'd have a basic ballgag. I called it the "Convertible Gag"

And a version of
that had a removable ball that could be replace with a metal ring.
Good times. Time to retool and do it again, I think!