ORIGINAL: Ed Smith
I would not screw the engine direct to the ply plate. The engine lugs are not a big enough surface to prevent then from "Bedding" in over time.
I would suggest a 3/32" or 1/8" thick aluminum plate about the same size as the ply plate for the engine to sit on.
Ed S
My first Dust mount was laminated with 1/64 ply and glass cloth top and bottom near the mounting holes. There was minimal mount maintenance after 100+ flights with a Jett 50. It held up much better then I expected it to. You can see the engine is still mounted after the final flight.