I want to keep it simple. Bolger's way of having two big pieces with a big connecting bolt makes sense. I had a rudder like this on my Windsprint.
I'm thinking of making the stock and the blade both with 1½" thick ply, and shaping the blade in a naca foil shape—at least, as much of a foil shape as one can get in 1½" ply and still have strength.
Here's the same concept on the rudder of one of Matt Layden's Enigma designs: