I just built one from scratch and documented it. I used nidacore. For another fuel hatch I used divinycel H80 foam. I think you can get nidacore in 2 x4 foot pieces from LBI or other suppliers. It won't rot or wick water until it weighs 80 lbs. Like my old fuel hatch cover before I replaced it.
http://www.classicseacraft.com/commu...ad.php?t=28820
You don't need to vacuum bag but peel ply is nice.
Also a repair on the hatch for the 25 with foam and no vacuum.
http://www.classicseacraft.com/commu...1&postcount=63
Good luck!