I used 8611 for rudder, 8411 for aileron, and 3421sa for elevator, they are probably overkill but it isnt necessary to worry about the weight penalty with this plane and I already had the servos. I am using the metal geared version for rudder. I think 8231 would be adequate, I would use the faster servo.

Or you could cut holes into the wing and surface mount the servos in trays and use the 9411.