On mine, which is definitely NOT portable, and is VERY solidly attached to the walls, I used sand for much of it. Plain "mortar sand", from a masonry supply, which, 15 years ago, sold for $20 a ton. Then the finish scenery, dirt, foam, etc, went over that. Foundations and roads were built up from whatever material seemed most useful at that locations, usually balsa or styrene, although a couple are plaster poured in balsa forms onsite, just like a concrete slab.
No, I didn't buy a ton. The boss and I couldn't figure out how to weigh a cardboard box on their truck scale, so we decided it probably held a little under 100 pounds, and rounded up to a dollar. I still have some of that box of sand, and found more at a remodeling project near where I worked. They dumped a big pile in the parking lot, and after finishing, left a small pile. I helped clean the lot...
