That definitely sounds promising. Might go with that. Simply boring out the plastic threads and gluing in a really strong magnet. Love the simplicity.
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my ball tops had metal threads. just a bit of advice if yours do. don't start with a drill bit that is the size of the threaded shaft. i did this and the threaded insert kept spinning and coming out of the ball top.
on my second one, i started with a smaller drill bit, moving the drill in small circles insid the insert. then i kept moving up in drill bit size, each small increment doing the same thing, small circles inside the insertuntil i got it boared out to the size i needed to push the magnet in.