Due to the torque you will need for both the rudder and elevator, I would make darned sure you attach the servo mounts to a hard point, not just skin balsa back there. Perhaps the light ply between two formers and a piece of 1/4" square hardwood on both ends where the servo mounting screws hold the servo in place (behind the balsa and CA'ed in place). Just attaching it to balsa, especially the thin skin balsa, is not a good hardpoint at all for holding in and securing servos.

CGr.