Hmmph, you're right, the plastic arm is too short. What were they thinking? Since you have a flat control surface, maybe you could just do without the base plates. I can't tell from your pictures (nice, by the way) but if you have a dowel/hardpoint there, you probably can do without it.

I have the same hardware included for the extreme flight yak I am building. The rudder is airfoiled and the baseplate will be tilted a little so there will be less material overhanging the hinge line. Still, I will have to move the horn back a little so that the plate fits. The hole in the plastic arm will be behind the hinge line, which I believe is OK as long as it's just a few mm's.

Peter