Hi Terry
having suggested the rigid foam backrest, I did then wonder whether squashed testicles might be an issue. The blocks on the boots solution has been done, although I've never seen it personally. There is usually a ready supply of duct tape around any gliding club to attach the blocks!
Essentially the older gliders are, the smaller the cockpit (pre-war pilots must have been tiny). Have you ever tried a K13 rather than a glass 2 seater? There are still a lot of clubs operating K13's.
The hand operated rudder modification can be found at some clubs - normally in a K21 but it's very expensive to fit so not common. The Walking on Air gliding for the disabled operation at Portmoak would be a bit far for you to go!