Not confident my servo setup would work, I put the motor in a blob of Blu-Tack, bent up the connecting wire and tested the set-up. And it worked!
The final box holding the servo in isn't pretty - I just stuck plastic rectangles around the Blu-Tack and servo servo with solvent and cement followed by 2-part epoxy - but it works. The servo isn't jumping around and it swings the rudder with gusto.
The rudder swings far enough to work, nothing seems to be catching on anything inside the model and all the working parts are under the removable cabin. I'm happy with that.