I am trying a Raboesch 50mm prop in a robbe kort nozzle. It seems that the 50mm prop may require fine tuning to ensure it does not bind. I am afraid I may have to run the grinder around the blades to get the correct fit after the final installation. But, while the rudder tube is still loose, there is no binding and it turns fine.
I like your use of micro-ballons to build up around the rudder tube. I'm thinking about doing the same with some sort of removable form.
Do you have any idea what kind of motor you will use for power? I tried the Titan 550 1:1 turning a Graupner 50mm 4-blade in a nozzle on my last boat. It moves pretty good but the motor will get quite warm. There is no easy way to cool the motor because of the kort nozzle. :'(
Keep up the good work!