When we bought Peter the captains cabin had a large and not especially useful built in sofa, which was using space that we would have preferred to use for bookshelves.
So out it cam, revealing gaping gaps in the panelling behind, straight through to the outer hull. Note the steering rods which run from the chains connected to the ship's wheel, right down to the rudder at the stern. Also note the wonderful use of ratty old carpet as makeshift insulation, genius.
The first step was to fill the holes in the wall, and insulate them while we were at it. Ever the recycler i used the slats from the sofa to make a little framework to hold a load of rockwool (we have LOTS of rockwool in bags in the fo'castle) before facing it with 25mm celotex. The reason the middle sheet of celotex has tape across the front of it is that the hatch and stairs down into the cabin is too small to get even a sheet this size down, so it had to come in 2 halves.
This was then covered over with a couple of sheets of 3mm mdf (which, fortunately, bends enough to go down the stairs!), meanwhile i measured everything up, got a whole load of 18mm mdf pieces cut to size at homebase, and painted them up in the spare room.
these were then screwed together, after a few trimming adjustments to account for the fact that there isn't a single straight corner, vertical or horizontal, in the room (you can see in the picture below that the top of the shelves, which is horizontal, is at a decidedly different angle to the windows behind it) and voila, a great set of bookshelves sprang into existance, meaning that we can have all our reference and art books in our office, hurrah!