Q1: can a plywood sheet of dimensions 0.75 x 48 x 120-inches be used to reinforce the cockpit deck on a GUARDIAN 19 and work better than using several layers of separate pieces of wood?
ASIDE: I have read several detailed threads on this topic, but none have stated if using a full sheet of 3/4" x 48" x 120" inch plwood and then tapering down the edges would work better than using several layers and several pieces of plywood together with epoxy to create the structure.
I would use epoxy to bond the single sheet of wood to the fiberglass shell. I would cover the wood with fiberglass on the underside.