The kind of locking system I was trying to make had another threaded ring on top of the ring you turn to change the spacing. This outer ring, when turned, applied compression to the turning ring by way of a tapered fit. The inner ring you turn to change the spacing has a slit in it to allow it to close up. I realised its design has some manufacturing challenges so a simpler idea would be to machine the turning ring on Stefans design, to be an interference fit on the threads, then cut a slit in it axially, allowing it to open up just enough for a firm fit. The thickness of this ring could be adjusted if more resistance is needed. Some testing will be needed to see what mass can be held comfortably by the part without tilt caused by the outer ring opening up under the load. an alternative would be to have two tabs machined into the outer ring, with the slit in between them, and a screw through the two tabs to provide locking of the outer ring.
All things to work with
