Tony,
I don’t know whether Porters website is up to date, but it mentions availability of both GRP and alloy tabernacles. But to be honest you could probably retain the wooden components you already have.
The only other thing I would add is to suggest that perhaps rather than glass the deck to the tabernacle, I would glass some battens/bearers to the underside of the deck “outside” the tabernacle and glue & screw the tabernacle to the new battens, so spreading any load. I am no expert however! Good luck with it…