You've got two choices. One is load the rudder so the boat always wants to turn one way, like tournament boats often have. That you do by shaping the rudder to pull to one side.
The second is to add an automotive damper like many manufacturers add to a car's rack. You could add it to the rack or the rudder. If you put a few mount holes on the rudder you could change the lever and make it adjustable. The thing about a boat is that it is not affected by small steering inputs immediately like a car is so the damper does not have as much effect.
I always took off the damper on any car I had that came with one. I just figure less work is less work.