There used to be a company that made hidden torque rods. They conected directly to the servo output shaft. they had 2 set screws to hold the torque rod to the output shaft cap. You had to put a flat spot on the torque shaft. The other end of the torque shaft was bent at about a 45 degree angle. The bent end would fit into a ply box that would go into a pocket in the control surface. I used them on a JHH F-100 and they didi not have any slop in them but for the life of me I can not remember what they were called or who made them.
Maybe someone here has used them in the past and can remember.

Joe L.