Ideas needed for wheel hangers

[nsa]

I made these wheel hangers at welding class but need some help with the finishing touch.





Behind the wheel is a metal plate that is fixed to the wall, and the tube is welded to the plate. I'm now looking for something to put onto the threaded bar in the centre of the hanger to stop the wheel falling off. I thought about buying a hockey puck, which is 3" diameter, and if I put a thread in the middle would fit nicely onto the M12 threaded bar. From memory the rubber used is quite thick so I should be able to tap a thread into it.

Any better ideas? Ideally whatever I have would fold back over the metal tube so that different size wheels/offsets can be accommodated, rather than just blocking off the end.

Criteria (assuming using threaded bar)

3" diameter or greater
M12 thread in the middle or ability to easily weld a suitable nut or tap a thread into it.
Easy to attach/remove

Thanks. If I have mixed up tap/die - apologies. It's new to me.