I made my gas tank hatch out of regular untreated plywood. Top sheet was 3/8ths or whatever the thickness is of the drop of the deck. Then I cut a piece 1" smaller all the way around and used some stainless screws to screw it on the underside of the top piece. This bottom piece (screwed and resined in) made the piece VERY strong. I rounded the edges with a router to make it nice but didn't need to. I also cut an access hole for the fuel sending unit (which I've ended up using a couple of times
)
Then I bought a gallon of marine resin from a Auto Body paint supply store down here (Ben's Paint Supply in Orlando). It was roughly $35 for the gallon AND hardener. I bought a fiberglass cloth the apporximate size of the lid (I had a few pieces to do so I bought extra) and trimmed it with scissors to fit nicely. Then I laid it all out and "painted" the mixed resin over the cloth and over the rest of the cover, one side at a time. Gave it a day or so per side to dry and I went back and gave both sides multiple coat to build some thickness.
The cover came out really nice. Cool thing is you can use cheap plywood to do it with since it will be waterproof. If you want to use nicer stuff you obviously can. I WOULD NOT use treated plywood though. Fiberglassing stuff is actually lots easier then I thought.
If anybody has any questions, I would be happy to help out.
The picture I am going to attempt to attach is of the traingle floor cover that I may for the front of my center console. Exactly the same process as the fule tank cover. Unfortunately I don't have any pictures of it while I was building it. Hopefully this helps though.