I had the thought of a pin through horizontally. Just wanted to see other clever ways people did it!Hi J,
Mine has the single screw too, but the tiller is wedged in so tight that I have never been able to move it.
If I had your problem, I would:
A: Drill a couple holes in the box channel and screw the thing in place.
B: Drill through the box channel horizontally for a trigger pin to hold it in place.
Option A is more permanent, but will produce a solid connection between the tiller and rudder head. Option B allows you to pull the pin for convenient disassembly or length adjustment. Also, option A is super easy and cheap. ;-) So, it depends on what you want.