>>10331722So it looks like the hull construction is a solid center piece that the hullform "ribs" slot into. That means you've got more slotted together than you necessarily need. So in the illustration, the gold line represents the windows. You could carve out some of the center where the two join and this should be deep enough to sell it without being so deep you cut all the way through the join. The green box is a small cabin, red is a larger. In either case, you'd want to essentiall make a box with a floor and three walls. Cut the walls to the height of the ribs. You may wish to add posts in the interior corners. This is not easy work, but if you really wanted a cabin in there then it is possible to do it and the resulting structure, thanks to a well-built box, would probably be an even stronger structure for the planking and decking. If you don't intend to light it, it doesn't need to be a lot in there.