A cheap a cheerful solution is just to weld an equal angle, say 11/2
inch on the the trailing edge of a flat plate rudder, as done by many
fishing boats. Try it with a tiller and gauge effect. Schilling
rudders are not good at making sternway. On my own vessel, I got
heavily into schilling rudders but eventually, after lengthy
discusions with the vessels designer of many beers, went for a
conventional NACA profile, which still has 70 - 0 - 70 rudder arcs.
Turning performance hard over at rest is pretty good, but would be
better had I had top and bottom plates added to direct/contain the
propwash.