3/4 plywood will work fine. Use exterior both sides good grade plywood. Use pressure treated 2X4s for the framing under it. Treat all surfaces of all the wood with fiberglass resin. Mix it according to the instructions on the package, then dilute it 50% with acetone and paint it on all surfaces. Give everything 3 coats, then a full strength undiluted coat to seal it. The acetone diluted coats will soak into the wood, then the acetone evaporates, leaving the wood impregnated with fiberglass resin. DO NOT use Thompson's water seal or any other product. Attach the deck to your framing (and assemble your framing) using stainless steel screws. After the final coat cures lightly sand the deck surface. Use outdoor carpet from Lowe's or Home Depot to carpet the deck. Carefully cut it to fit your deck. Then spread outdoor carpet adhesive and apply your carpet. Use a roller to smooth it out and avoid bubbles. I used a wooden rolling pin for baking as my roller when I did my boat restoration. It works well for small jobs.If you follow my instruction you will have a deck that will last many years with no problems whatsoever. Enjoy your casting deck!