This is all very good info. Thanks for the input. I can appreciate some of the comments, such as getting a consistent foil shape out of the aluminum and the warpage issue on wide wood planks.
I did think about using a layer of 1/2 inch plywood in between the solid pine in order to help the overall strength.
I didn't think about the floating issue and have wondered about keeping such a system in the down position. I thought I would rig a rope to tie it down, and another to hoist the rudder up when needed. One concern is allowing the rudder to pivot up in the event I got into the shallows. It needs some sort of shear pin type setup-maybe a wooden shear pin that keeps the rudder down but would break if stressed by a sand bar. A weight sounds much better, no fussing with a pin.
I wish I lived near a marina so I could go look at some samples. I'll keep kicking this around until I settle on a good plan. No money to waste on experiments (unless they're free!) Flat.