When we bought Irish Eyes the two water tank covers were gray plastic. The holding tank cover was plywood with fiberglass on its interior surface. I replaced the plywood holding tank cover with 1/2" gray PVC that I bought from McMaster Carr. It was identical in appearance to the existing water tank covers, so I assume the water tank covers are PVC.
The forward water tank cover has a single 4" Beckson inspection plate in it. The aft water tank cover has pair of inspection plates. All three of the holes for the inspection plates were reinforced around their edges by having a approximately 12"x12" piece of the same gray plastic with a hole in its center glued to the underside of the water tank cover with the hole in the cover and hole in the reinforcement lining up. You can feel and see the reinforcements through the open inspection plates.
My guess is that the reinforcements were glued in place with standard PVC cement after the two surfaces were cleaned with PVC cleaner. Both should be safe for water contact as both are routinely used in house plumbing. C-clamps through the hole should hold the reinforcement in place while the glue hardens.
I did not add the second thickness of PVC to my holding tank top.
1988 PSC 34
Irish Eyes to the Bahamas