Mick,

I built this three years ago. I now recall I put down a base coat of Plaster of Paris for rigidity, and smoothed it with the drywall mud. There was no problem getting the plaster to adhere to the styrofoam. The plug was 14 feet by 24 inches beam, a separate piece for the deck and another for the hull.
The hull was light-weight enough to be moved easily. The deck plug was a lot thinner in depth and wanted to flex around the cockpit when lifted, and could have benefited from some type of rigid armature. When I was painting and final sanding the plug I had a little trouble cutting down to the pink styrofoam in places, easily patched by digging out the exposed foam and filling with mud. If you have a choice of types of mud get the lightweight variety, as it dries a lot quicker and is easier to work.