This should work on those big barrel lenses you pick up cheap on eBAy but never seem to come with a flange or jam nut. Use a tank to bowl toilet gasket---a big heavy tapered rubber washer. It should work on lenses a bit larger that 3" in diameter. I mounted a 178mm Aero Ektar on a "C" board for my Speeder and it works! Thrifco part # 525T. They also make a slightly smaller version.
Cheers!