There are other downsides to the stock rudder that make a replacement worthwhile. First, if you look at it carefully, you will see that right where the water hits it below the keel are several square inches of a flat (!) surface acting like a brake (the tapered edge starts a few inches lower). Second, the cheeks between which the rudder blade pivots also extend lower than the keel, and are therefore dragged through the water. Third, it's quite heavy and unnecessary weight off the transom isn't a good thing.
There's an old article floating aroundthat gives dimensions and a design for a rudder using an aluminum rudder head that fixes these issues. From my vantage point, the downside of this design is the use of aluminum. I don't like the looks of it particularly and I don't have tools or experience. Further, it's designed for a fixed tiller (mine tilts up) and for a different placement of gudgeons.
So I decided to adapt it to my purposes and went back to a rudder head more like the one on the stock rudder.
As material I'm using 2 sheets of .5" plywood, glued back to back. For the foil, I used the layers of the plywood (there are about 8 of them in each sheet) as depth gauges. In other words, I marked the expected pattern of dark and light bands that I would get from sanding the plywood into the foil shape.
That part is done, and has worked out reasonably well. A power plane and a good belt sander with 40 grit worked well to get the shape roughed out and keeping the pattern symmetric and straight gave me a good approximation of the final foil shape.
Lots of fine sanding and some applications of epoxy fairing compound later, and I was ready to sheathe the blade in a lightweight glass fabric (3.7 oz.).
That's how far the project has progressed for now. The rudder head is still connected to the the blade. That made working on the latter much easier, since I have an area to clamp on to when working on the blade. It also makes sure that the two are the same width, so after I cut them apart, I can glue on the "cheeks" and the blade will fit between.
Read to the end of this thread for some updates based on multi-season experience with this design.