'Mosgummi' is a tradename in Denmark - if anyone knows the real/english name please tell me.
This material is kind of a cross of rubbery and foam, it bends and can be easily cut with a knife or with a hot-cutter
Mosgummi can be bend within reason and spring back into form again and besides cutting, it is dificult to harm. The only downfall is that is don't take paint well, so you are limited to what colour you can get. This should not be too much of a problem as I would only suggest using mosgummi to make an 'emergency travel set': something you can cram into your backpack when gaming somewhere without much terrain.
Assuming you don't have a car to drive in.
The picture more or less shows how to make ruined corners, but I'll just go over the process anyway.
Cut some stripes of mosgummi that are as wide as you want your 1-story buildings to be high (I assume you only want 1-story buildings as that is by far easiset to make). A shart knife works best here.
Each stripe should give you two corners.
Cut your stripe into one long and one short parts to to make corners that have a long and a short wall with diferent looks.
Using a hotwire-cutter (it is slower that cutting foam) or a knife, make a cut in each stripe.
Now take one cutoff from each part of the stripe and lay them on top of each other alligned at the bottom and the vertical end. Use a knife to make some cuts trough both pieces at the same time. This cuts should be as long as the pieces are thick. The last item to do is to cut between the cuts so you have 2 walls that can be asembled into one corner.
The pieces will hold nicely together and the corners will look diferently as the pieces that was cut together belong to different corners.
For travel the pieces can be laid like a jigsaw. Then they will not take more room than the rulebook.