After the warping incident I set out to test a few, clearly more succesful, ways of basing terrain.
Since I only had foamboard, I decided to try out, what seemed to a general consensus of several pieces of advice I received, and simply paint both sides of the board. Only had 3 mm left, but I figured tat would be the ultimate test then. So before anything else I painted the foamboard with just a thing layer of a cheap artshop black acrylic paint.
By suggestion I had gotten some foamex - or foamalux, as it was called at my local signmaker's. Quite easy to cut, and not bendy once it's cut down to small bits like this one.
Both then had a bit of details added, and then a mix of filler, black paint and various sized of gravel.
They have now dried over night and look mom - NO WARPING!!!