How about a scale-like setup. Cheap, easy, weightless.
Come off the rudder horns (2 required- as in pull-pull), with stranded fishing line.
Go forward to the rear float strut,
Go down to the top of the float,
Back to the water-rudder.
Pulleys are not required. I use 2-56 eyebolts.
this is a full-size cub at Jack Brown's seaplane base. You will find that the cable has to be crossed once for the water rudder to move in the same direction as the air rudder.