Eelco, A lot of home improvement stores that sell sheet foam insulation have pieces that are broken. If they are broken to much to be sold they will usally give away for the asking. I have associates at some of the local improvement stores save the broken ones for me. Might be worth a try. I like to use plaster cloth over foam for a base. Paint the base with an earth color and while it is still wet sprinkle on sifted dirt or what ever you are using for ground cover.