A few weeks ago at work I discovered that they were throwing away large sheets of polystyrene, used as transport packing... about 1200mm square and 70mm thick. Gloriously thick stuff.
So, after asking, I started grabbing it. :)
Now, all I have to do is (a) store it and (b) find a worthy use for it... and keep my eyes open for more.
I've already rebuilt my hot wire cutter and started cutting it up into 600mm squares.
I have visions of Omaha beach bluffs... and point du Hoc.