Have you thought about using expanding foam insulation? I'm sold on that stuff from now on... You simply apply it, then sculpt it into drifts as it hardens, fine-tune its shape after it dries, paint it white, and then apply whatever "Fresh Snow" material suits ya... Drives Doog nuts, but I use baking soda more often than not because it has that "sparkle" when the light hits it, and only has to applied in a VERY thin layer...
Also, the foam is easily cut with a knife or "drilled" with a skewer for insertion of trees/brush. telegraph poles, digging foxholes, footprints, vehicle tracks, whatever.. You don't need a foam-cutter to shape it, either... You can get it at about any hardware store...
I used it here on the Flak 36 WIP. Although this'll be covered in grass & dirt, it shows how you can shape it and raise your "ground" on a flat base, rather than have to actually "Dig" down, or glue foam blocks onto a base. I use plastic photo box-frames (8 x 10 here) in order to get max depth and it also is really light-weight...
Once it's where I want it, I'll just wrap it with wood veneer...