Adding a deck extension

[geno27]

I have a 1986 Winner bass boat that I am considering adding a deck extension to (and maybe a storage compartment built into the extension) before I re-carpet or put Durabak down. What is the best material to use for this application? Will I need to treat the material? If so what's best to use?
Thanks for any help.

[cyclone70]

I made one with a cooler and speakers for my stereo built into it.
Made most of it out of 1/2" plywood and coated the whole thing with F/G resin before carpeting the outside.
The four vertical supports that the plywood screw to are made out of P.T. lumber.
The thing is as solid as a rock.