The Revell Type VII C U-Boat kits are both full hull models. I made my five U-Boats as waterline models so I could wargame with them. That left me with a bunch of lower hulls. One of them I flipped upside down and made into a turn of the century submarine. This one I made as a play set submarine. I cut down some of the sides and constructed an interior. I took several of the interior supports from a couple of the subs and used them as interior dividers.
I then took strips of styrene plastic as hull supports. I put about one every inch or so. One of my goals with this project was to spend the least amount of money possible on the construction. I also wanted this submarine to have good play value, so it has to be roomier than the original in order to move the crew around inside.