hi there TT
I have solved a similar problem on a model Air Sea Rescue Launch 2564 - the only difference being there are two rudders slung over the transom. The tiller arm goes through two slots cut in the transom, the slots in the transom are very thin only about 3mm-4mm wide and about 10mm-15mm long.
If you have a look at the drawing you will see an extension arm, which has a slot cut into it. The pin that goes through this slot, which is connected to the 'dummy tiller arm' has to be pretty 'neat' to stop 'side-slop movement'.
On one of the drawings you will see I have drawn the pin mid-way in the slot, when the rudder is facing fore and aft. This is 'technically wrong' the pin should actually be very close to the top of the slot next to the servo - I have only drawn it like that to illustrate it for you
The other thing is, the material that you use to construct the extension arm, with the slot in, try and use aluminium, brass or a very hard material. Try and avoid using plastic, as my original one was made from plastic
and after a number of outings - it snapped .
This is an idea for you anyway my friend. Gives you food for thought.
I did try and take a photograph of the Air Sea Launch but, they are actually mounted right against the stern, and a little difficult to photograph through the hatch, I will give it another try though and see if I can
aye
John e
Bluebird