Gary - your photos pretty much mimick my written description. Replace your polyester filler with plaster and it’s pretty much the same concept/practice. Thanks for photos thqt help explain my text.
I think I came up with a method. I used a mixture of bee’s wax and paraffin instead of cement. It’s easier to scrape, it’s repairable if you screw up(don’t ask) and obviously you don’t have to wax it. I used 1/8" wire as the rails and as you can see you can do difficult shapes. If I was doing more than one I would do the positive/negative thing to arrive at a permanent mould. I filed the scaper out of 1/16" aluminum.
One thing, I haven’t actually laid up a rudder yet but I don’t see any problems. The picture is of a test. The real thing will have level surfaces on the outside of the wire.
Don