Dont wait for someone from the blue to come and help you with this one. THING is a very little known vintage here in Pakistan so the chances to get a wingnut/clamp are meak. Take the driving seat, make use of google, find out the dimensions/sizes of the wingnut/clamp and get one fabricated (if not possible to import).
If you cant find a fabricator to do the job for you, drop me the research/measurements/specs. I would get one fabricated here in Multan for you from Ustaad Hashim.