So, I've decided to try a bit of DAS air-drying clay:
While I was at the art supply store, I saw some artist's panels. It's a very thin sheet of canvas bonded to a piece of what appears to be 3mm (1/8") MDF that's been sanded and stained:
The price is very reasonable and it comes in four different sizes. This one is 12"x16" (30.5cm x 40.6cm) for $5. That turns out cheaper than the sheet styrene I've been contemplating!! It will need a blast of polyurethane wood sealant to ensure it won't warp, but this thing has real terrain possibilities!! I'm still left with the dilemma of how to create different ground levels, but at least it's a cheaper option.
PostScript: Now that the clay is dry, it just doesn't seem to hold on too well. So green goop it is.