Denise is amazingly helpful, isn't she?
From the plans Denise posted it looks like the C/B for your Sailmaster is roughly rectangular in shape. If you have access to the C/B trunk on the boat you can even measure the inside dimensions of the trunk. I'd start with a plywood prototype that could be covered in
epoxy and cloth or just used to give a metal fabricator.
As Denise also noted it is rather important how heavy the C/B was intended to be. If it is supposed to be over 100 lbs. a light weight plywood,
epoxy encased version might not stay down as it should. In that case I'd take the plywood prototype to a metal fabricator and have them make a metal core out of 1/4" or 3/8" stock that could also be covered in
epoxy and cloth.
My 27' S&S designed Tartan also has a C/B and is of similar hull shape and our C/B weighs somewhere in the neighborhood of 100 - 150 lbs. I believe.
A plywood version could be made such that some lead inserts were included to add weight where it is needed.