Ok, when you mean make a box just piece off wood screwed to the sides an back sand down an float
Glued, rather than screwed. This tends to be more water resistant, which is a good thing on a boat. Paint can also be useful for keeping the water on the outside, although dougal99 did say "starting with a framework and then sheeting the sides". The articles that accompany the free plans usually give a plentiful supply of hints and tips to get any beginner who cares to read them into building simplified hulls.
Despite you being unable to find precise instructions for the exact hull that you want, the fact remains that the simple box-type hulls all follow the same general method, you know one, you know them all. Its just the sizes of the panels that changes. Adding your chosen details to make it into the ship you want is another story.