This is an interesting predicament.
I bought over half a dozen happ trackballs from a guy on KLOV for $6 apiece.
Thought it'd be easy to find the mount, but after a couple weeks of confusing emails with parts websites and a phone call with arcadeshop.....I friggin' gave up.
took a 1/4 inch piece of aspen wood...cut a 3 inch hole...cut a big hole in the panel so the trackball slipped in...routed the panel for the aspen board...and then bolted the trackball to the inlayed aspen wood.
works great.
it's silly you can't buy a simple retrofit trackball mounting kit out there.
apparently all trackballs back in the day were bolted to metal panels, like a centipede cabaret?