Graham. It is possible to carve a hull out of foam and cover with balsa but depending on how much foam you want to carve out it could be very week. Unfortunately although this would get you a big hull fairly quickly, it would need a lot of ballast and a lot of foam would need cutting out.
Better idea would be to cover the foam in fibreglass/resin. This would produce a strong hull, but would require extensive work on the outside afterwards.
I do not have an Illustrious plan, as the model was developed from the carpet carrier made earlier, and was sized for convenience and as a minimum to get the die cast aircraft to look all right on the deck.
Ashley