Just testing this still works. ;)
I needed a shim between the Fujinon fisheye and my one piece C mount to Pentax Q adaptor.
Re-using the black rubber o-ring seal from the silver metal adaptor out of the Axis security camera was very handy. It is offering just the right flex to allow for smooth focus adjust that has enough resistance that stays put when I want it to. Acts like a makeshift helicoid I suppose. Give it a try if you did not throw out your o-ring already. Otherwise a similar size o-ring would do the trick.