In the rules, you start off with a small vessel of two guns which is supposed to be no more than 6'' to 8'' inches long. I thought this was a bit on the tiddly side so mine ended with a hull of 9'' and a deck width of 3'' at widest point.
As I had a load of left over balsa wood from a school project a few years ago, I decided to sandwich three layers together to produce the basic shape, before sanding it down into a nice smooth hull shape. Next I'll add cardboard sides to the hull and stern, together with decking out of scored card or wood strips.
I'll also cut two gunports in either side of the hull for the cannon, which I already have in my spares box. I think I'll stagger them, so that one gun is fore and one aft on either side of the deck, giving me plenty of space to place the crew figures.