I built the hull of my Wren out of 2 sheets of 3mm plywood. It's 18' long. I am glassing the outside to add strength. I also added a layer in the cockpit for abrasion and was surprised at the strength... well, really it's stiffness.

It would take at least one more sheet to do a deck (I did a strip deck)and a small piece of scrap too. It would tight but I think I could make one from 3 sheets with some good planning.