The top of the tank is also a quarter berth... a mattress gets thrown up there and a body goes on top of the mattress... I'm thinking perhaps 1/2 inch... but that might be overkill. Maybe 3/8 of an inch thick would do it.
I really like the idea of a fully inspect-able tank. Of course I'd cut an access port into it.
It just seems that Lexan or some other plastic would be vastly more durable than marine grade plywood that has been treated with epoxy resin. Yes, at a higher cost, no doubt. Of course, if I get a good sized piece of Lexan, I can also make a clear hatchboard to replace one of my solid wood boards... thus offering me a "window."
Its about 28 inches wide by 48 inches long... long being along the axis that one would lay. Bit of a curve in one area... it tapers somewhat, becomes narrower aft.
The real irony is that in looking to fix this... I found an access hatch I didn't know existed.