Once it's based, it's surprisingly durable. I recommend hardboard or mdf. Do not use card or foamcard. They are lightweights, and it's hard enough to keep them on the table without some weight in the basing.
They are rather thin vac-formed mouldings, a little bland (which is good, as you can always up-bling something - it's a lot harder to de-skullify) but adequately made (and miles better than GWs last attempt at vacformed terrain pieces.).
For extra rigidity, you can fill the voids with expanding foam.
Hey! Where'd ... Everybody ... go?