Would it be possible to turn up an alum. driver and put a small keyway in the crank and driver?
Maybe a shoulder on the crank to stop the driver touching the crankcase, or just a driver and set screw like this. Maybe 2 set screws to balance it all up. Might need to "dimple" the crank to stop the driver sliding back on the crank when the prop nut is tightened or just machine a shallow groove around the crank for the grub screws to seat into. Maybe run the screw right through the crank? A small split pin even. I seem to recall owning an engine that had a split pin through the driver, might have been an early FROG 80 diesel. Don't know if Zagnut meant this type of thing?