Hi Gary,
Very sensible to experiment in plywood. If you start with the largest endplate you've imagined, you can trim it next time she's out, seal it up and leave it in place. If it turns out too big, that is. I don't know how you'll tell...
If the rudder is 4cm thick and I were going to do it, I'd make the endplate, oh, about 26cm wide overall, leading end flush with the rudder's start, trailing end rounded off somewhat aft of the blade's finish, all in a fair shape to shed weed and fishing lines. That should be big enough to test the concept. All guesses!
I'm not hopeful that an endplate will help your weather helm much, but a significantly larger rudder will. To my eye and Arne's, Redwing's is small.
Be brave. Experiment with an added lower section, which can be sacrificial like ours (in technical illustrations.) So... another guess! - try adding one-third more to the area in the water. Just carve up a chunk of timber for the first edition. For attachment, Sikaflex is pretty good stuff. Add some balance to the lower part to keep tiller forces light. You'll have to satisfy yourself that the structure above can handle it.
(I can't remember whether your keel lifts or not. If so, I'm sure you've seen many trailer-sailers with arrangements that allow for lifting the rudder foil too. There's a project!)
A few degrees of weather helm (tiller angle off centreline) is good. Most junks don't need to reef in 15 knots. Were you heeled way over? There are other ways to attack weather helm, as you can read in Hasler & McLeod and elsewhere...
(At this point, somebody will expect me to suggest you take the camber out of your sail, so I'll suggest it. But don't.)
After our experience of putting up with a poor and breaking original rudder, then making do with a prototype that we couldn't step away from, and finally getting it roughly right, I'd say your efforts in this area will pay off.
Cheers,
Kurt