If you don't mind doing surgery on the existing rudder, you could drill some lightening holes thru it, then wrap the outside in glass or carbon fiber mat, with epoxy. Light and very stiff is using the carbon fiber. ?If carbon fiber mat costs too much, you can use carbon fiber tape, laid out in strips, almost as good. They might even act as turbulators, unless your overcoat of epoxy is well-faired.
