Placement of fuel cap/entry in cockpit or on boat edge and material for replacing fuel tank support

Greetings, I will be replacing the rotting plywood that supports the fuel tank and am considering materials. I remember someone in the sailing club saying something about mold-resistant plasticized plywood but don't remember who said that. Does anyone know exactly what that is called and where that might be sold in town? Or have suggestions of other approaches to a replacement support?  I am actually unsure of the capacity of the fuel tank. It isn't large. Specs for the boat said the original tank was 20 gallons. Looks likely original, but a lot of people who have looked at it think it must be smaller than 20 gallons.

Also, I am mulling whether to move the fuel cap (fuel intake).  There is an unused intake on the boat edge.The previous owner though, had put an intake direcctly over the tank in the cockpit floor. That is clearly not sealing properly, so something needs to be done. I an interested on hearing preferences for location of the fuel intake. The cockpit intake creates a very short direct trip to the fuel tank and would be easy to fill at sea if need be, but seems to raise the risk of water into the fuel. Using the boat edge intake would mean running a longer line and that would be sort of in the way of an edge of the storage space in the only real lazarette/locker in the cockpit.Obviously, it would have to be somehow protected and it would be a longer trip to the fuel tank for the fuel. Other pros and cons?

I actually have someone ready to start helping me dig into this project in a couple of days. (Yay!)  It's probably wishful thinking, but I'm hoping this project will be done by late next week

As always thanks for your ideas.

Patti (the Alula)