Concerning foam, extruded polystyrene foam boards should be used (XPS), not expanded polystyrene. Expanded polystyrene foam will crumble when being worked on, XPS will not.
I also intend to make terrain for Battletech out of XPS boards, and I have a question related to that - what would be the most appropriate thickness of XPS boards for modelling terrain, 2cm or 3cm (for 1 level height)?
In my opinion, 2.5cm (1inch) would be ideal, but I live in Europe, and all I can find are 1 cm incremental thicknesses.