From what I can tell the panel sitting near the trailing edge is normally fitted near the leading edge just behind the cannon fairings - and the other two double-hinged panels swing aft to cover the magazines and rear portions of the guns.
There was a discussion about this on Britmodeller and whilst nobody seemed sure I think it might have been Chris Thomas, whose opinion I would respect, who thought it might have been some kind of fibre board.
On a model I think it would be a fairly simple matter to represent the material in a realistic way without having to know its exact composition, be it fibre, leather or indeed felt.
Cheers
Steve