Typically rudders on production boats have a stainless steel
tube running the length of the rotational axis to which flat bar stock is welded to in the shape of a frame.
is made from which the rudder halves are layed up in fiberglass. The halves are bonded together around the stainless steel
frame and the void filled with foam.
Alternately, with custom boats I have seen a bulk of plywood
laminated around the welded rudder stock and frame.
This mass is sculpted into shape using a planer and rasps to get close to the final shape, then coated with epoxy
and fiberglass before a final fairing to get to the finished form.
The second method saves on the labor of making plug
(usually foam) and a mold
, but the first allows for a much more accurate finished form.
Either way, most are using templates of the profile to gauge the shape, make two sets where one is slightly oversized. This oversized set will help you see where you need to remove material. The one that is the right size is for checking your results and/or adding material if you removed too much.
Good luck, show us some progress photos!