Ditto on what beerjet said... I used regular ply.. either mdx or cdx(come in one side a bit smoother when I got mine). Just seal and glass it. I even went over with gel coat. If you are looking for a lighter way to go and don't mind the cost... go down a to a fiberglass material store, they have composite sheets in different thicknesses. Its some sort of poly/air combo and super strong. After you add the glassing your back to the weight though.
