This is perfect timing as I'm about to embark on a similar project. I'm also going to turn my 23's flat plat to a NACA 0014 foil only instead of foam, I plan on using a series of foil shaped blocks and a hardwood leading edge just as you are. The "framing" will be wrapped in an 1/8" mahogany plywood and fiberglassed, giving the look of a wood foil rudder. My son, who builds surf boards, will fiberglass it for me- he's a better glasser than I.
I plan on chronicling the process but, as of right now, don't know how to post photos here. I guess this is a good time to learn!