I would think that an 8" square pan should suffice. Bake 2 x Madeira cakes.
If you level off one, add some filling/buttercream and then put the other on top, that should provide something you could work with. You could cut a 2" strip off the front of the assembled cake, then carve the top to chamfer it (make it treasure chest shape).
Then ice it.
The madeira cakes are the easy bits:
Each cake :
4 medium eggs, 8 ozs SR Flour, 4 ozs Plain Flour, 8 ozs Butter, 8 ozs Caster Sugar, zest of 4 lemons and 4 tsp lemon juice.
Cream the butter & sugar, beat eggs and add slowly, mixing thoroughly. Add zest & lemon juice, then fold in flour. Spoon the mix in to a greased and floured pan and put in a pre-heated oven at Gas 3 (160deg.C) for 45 - 50 mins.
If you can get a pan the divides to two 8" x 6" sections, then you could reduce the mix by 25% for each cake and bake at the same time. That would remove the need to cut of the 2" strip. You could get that from Lakeland or your nearest good cookshop.
I can't help you with the icing, save to suggest googling "Fondant Icing". Renshaw are very good at that sort of thing, or Squires Kitchen.