John K Jordan
I have made knife cuts up the inside of the hex hole which tightened up the connection for at least a while. I wonder if a thin coat of epoxy or epoxy paint on the top of the shaft would enlarge it enough to provide friction.

A wild idea I haven't tried: mount a ring of little supermagnets in holes around each of the mating surfaces. Would make for an interesting operating experience but might keep the thing together.

I agree wear or enlargement of the nylon insert is a problem. Perhaps could be designed wit small leaf springs or better, spring-loaded ball bearings in the upper part instead of relying on the friction of the nylon insert.

Maybe one of these would do the trick. Before assembly drill and tap a hole in the side from the outside, insert the ball plunger, then if necessary file a shallow recess around the hex shaft for a positive detent.

Just brainstorming...

JKJ