Nobody replied? I fixed a number of rudders, some new ones even before they split as you describe. I sanded as flat as possible then epoxied on a strip of 25 mm Kevlar woven tape.
The rudder "cases" want to split because of the bending one way then the other.

My current problem is redrilling rudders to make rudder pressure versus raked mast correct. I cannot find redrilling specs in this thread. My rudders are still too far aft.