The hard stuff is better if you can find it (more robust for regular use) but all of my stuff is just cheap white polystyrene used in packaging .... pulled from a skip! I have stucc it to 3mm mDF to make it more durable. I use whatever PVA (wood glue) I can find going cheap ...for about 90% of all scenery projects.
Would PVA glue hold, say, a Renedra plastic tent to the Styrofoam?
Regarding the MDF bases, were they custom made into kidney shapes? I searched for some that would be suitable for terrain, but haven't found anything suitable.