[JA]

I just looked at a thread on cloudy nights. Now I see the construction: a rubber sleeve over an apparently 0.515 inch diameter brass tube.
At a pinch you could use a short piece, say a one inch long piece of 1/2 inch diameter (ID) heater hose from an auto parts store. You could also use the opportunity to increase the outer diameter of the knob substantially to assist with finer focus adjustment.

Best
JA