-- Last Updated: May-05-08 6:11 PM EST --
I'm not obsessed with things being neat and tidy, but I dislike stuff that's cobbled together. I'd make the whole she-bang from metal, with the possible exception of the rudder itself. The method I'm thinking of would probably weigh about three pounds if made out of appropriately-sized steel stock (steel is great material because it's easily welded), and much less if made from aluminum (unfortunately, only "pros" can weld aluminum). What I envision is like this:
The rudder itself would have a vertical pivot shaft within a housing that's about about 12 inches high, positioned directly behind the center of the stern, with the bottom end right at the waterline. This would just be a round rod inside of a tube, with a stopper at the top and bottom to keep the center rod from shifting up and down.
The part that attaches to the boat would consist of two sections of angle stock, one on each gunwale, located just forward of the rear deck plate. Each of these would also be roughly one foot long. These angle-stock pieces would have one leg of the angle on top of the gunwale, and the other leg on the outside, so 1.5" x 1.5" angle stock would be a good size (if steel, the thinner the stock metal the better). They would attach by bolting through the gunwale. I'd connect the two gunwale mounts with one, perhaps two, cross pieces, in the manner of thwarts (two would be better than one, but one would probably be enough).
The connection between the gunwale mounts and the pivot housing is next. For that, I'd use either square stock or angle stock (1" x 1" if aluminum, 1/2" x 1/2" if steel), with one pair of connectors going from the rear of each gunwale mount to the top of the pivot housing, and another pair going from the front end of each gunwale mount to the bottom of the pivot housing. If the construction material is as light as possible, placing a diagonal brace between each pair of gunwale-to-rudder connectors would be a good idea.
The final touch would be to mount the vertical pivot housing to a matching vertical bracket, rather than directly to the four connectors leading to the gunwale mounts. The pivot housing would attach to that matching vertical bracket with a horizontal pin at the top, and just a wrap-around piece of low-strength coil spring at the bottom (you could also substitute a weak "shear pin" for the spring). That way, if you hit a rock while underway, the pivot housing would tilt up like an outboard motor, rather than wrecking something.