tiller

Maybe run a line across the boat from one gunwale to the other, under tension, on the way wrapping it once, twice or more around the tiller? Would tend to damp out any movement.

There's always this solution:

http://www.rwo-marine.com/tillerbrake.html

but I'm not sure how useful it would be in a dinghy.

Maybe some food for thought though...

Why not contact the Buzz class association, explain the situation and see if you can convert from a single-tiller extension to twin tillers extensions? The advantage to the twin tiller extensions is that they are mounted to the side of the tiller and not on the top and with rubber joints, they should sit naturally at a position which your dad will find them easily at and it also means the extension doesn't need to be flicked across the boat. Make sure the extensions are long enough that they won't get caught inside the boat though. Thick shock-chord limiting movement of the rudder does work to a certain degree, reckon if it's looped up through the bottom gudgeon and over the top of the tiller directly verically above it, very tightly, it restricts movement of the rudder blade very well. Seemed to work on training sessions on our boat but we have a lighter and probably smaller rudder blade than on the buzz.