That Urethane foam would work great for an Island, you could make whatever shape you wanted out of cardboard, spray a release agent on it and seal it with tape then pour it in. You can also shape it by using a hot wire, that's the way some composite aircraft are built.
I've used it to fill cavities in boats and it's amazing how much it expands.
http://www.youtube.com/watch?v=HxXgdInnpLw&feature=player_embedded
