that was part of the reason I thought adding a slot in the center might help. It's not the ultimate solution but we could make a double sided plastic roller that would stick out of the sides of the rudder (kinda like frankenstein's bolts in his neck) and fit in that slot. One simple hole drilled in the rudder will do. They would take the majority of the frictional load and make rudder shape less of a problem. However, I'm not sure the slot could be made as long enough (it would need to be as long as the vertical travel of the rudder). In any case, something somewhere would still have to be shimmed to fit - I'll continue this thought next time I'm in my design office (the bathroom).

I literally have a folder with various toilet paper and restaraunt napkin scriblings in it! (was that too much information? )