We found a way to build rudders and keels in a warp-free and asymmetric problems-free manner.
Briefly, you need to draw and cut the side view of your keel-rudder from 0.8-mm ply, once done join both halves with sellotape, (the brown package tape is perfect.) First on one side, lay the leading edge, and then, after sanding for perfect match between the two halves, the other edge, in our case the trailing one.
You now have built a sort of "envelope", and like an envelope you can now open it and fit a brass rod in the case of a rudder, or a balsa "spacer" as to give it the correct shape, slightly wetted with 5 minute epoxy glue.
You can now fill it with resin if you wish or just let a little quantity of resin to run on the inside edges of the "envelope". Then close the top in the rudder case, or both sides in the case of a keel with a mixture of epoxy and micro-balloons, or just with a piece of timber.
When set it is done, take the sellotape out and a new rudder-keel was born! For perfect similarity (does this word exists?) it is important to have both halves identical, when you join the two halves on one side first, then sanding and then sellotaping the other edge. It takes less than ten minutes to have the rudder or keel PERFECTLY shaped.
A variation we use for building keels and rudders is to use fiberglass instead of plywood. We lay a sheet of fiberglass on a glass surface, the quantity of fiberglass (thickness and materials according to forces involved, you can also use carbon fiber to prepare the sheet.) Then using it, as before instead of the plywood, it will have one side (the one that was on impregnated on the glass) for a perfect finished, and now ready to be glued. Once you are accustomed to the system you can prepare a few sheets and keep them at hand, it will then take you less than nothing to build the new shaped rudder and have it ready for the pond in the afternoon!
We used this system for building a large number of rudders, and many keels, some of which for the A Class, that is to say they hold 10 Kg! without any problem. Hope this helps someone.