Plywood rudders work quite well for pindle and gudgeon hung rudders with the alternating plys distributing the loads both vertically and horizontally. They should be shaped to get decent flow and beefed up above the waterline so that the torque is fully distributed into the foil. They do need to be sealed with epoxy and ideally with a layer of cloth.
For any sizable boat the rudder would need to have a thickness of roughly 1.5" (40mm) to 2.5" (60mm) thick. And there is the problem-properly engineered it would be extremely heavy and difficult to ship. Beyond that the pindles and gudgeons would need to be very sturdy and the transom beefed up to take the huge loads.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies