I started with a basic shape cut from styrofoam, using a plastic cheese spread container for the conning tower. Figures should be able to stand in the conning tower.
The deck gun (from anyscalemodels) was glued to a magnet so it could be easily attached and removed.
To achieve the riveted plating look I searched the internet for suitable pictures, stitched these together to create a larger surface and printed several sheets.
After covering the hull with the paper (note: this did warp the styrofoam, making it necessary to leave it for a while with some weights on it to counter the warping) I delved into my bits box for detailing the hull and conning tower.
The aft gun (from Tiger miniatures) was glued to a large nail so it could be removed. The railing is made from chicken wire.
The end result after painting (rivets and lines on the conning tower were done using a sharpie):