Best material for building boxes for fuel tanks

I had to remove the old wood flooring, the steel fuel tanks and steel water tanks from the cockpit of my boat (The wood rotted and subsequently the tanks rusted). This has left me with a space about 12' wide by 5' long by 2.5' high. After considering my options (buy aluminum, build my own or buy poly tanks) I have decided on multiple 50 gal poly tanks. This will give me enormous flexibility and ease of service. I would like to surround each pair of tanks in an enclosure that will support the tanks and have a sealed bottom in case of a leak or spill. I could use 3/4 inch marine ply coated with cloth and resin, but I would like advice if anyone has opinions regarding a simpler solution: use plastic or composite board and only tape the seams. Which would be best for this? Coosa, Starboard, etc. I even thought of using hardibacker, but I don't think it will stand up to the vibration. The boxes will not have tops, but instead the sides will act as part of the support structure for the new cockpit floor. Removable hatch panels will be screwed and gasketed in place in case I need to access the tanks. The fill and valves will be in there own water-tight and vented compartment for easy access.
I am starting off with one pair of tanks while I continue my refit work, but ultimately, I can fit a total of 300 gallons (the size of the previous steel tanks).

 

Can you, should you, build your own fiberglass tanks? By Ken Hankinson

http://www.practical-sailor.com/newspics/charts/8912byofiberglasstanks.pdf

 

Gasoline or Diesel?