My last snowmobile-camping trip, I took a full-length closed cell pad and a short thermarest. The Thermarest it turned out, had a hole (#!@# cat!) and went flat in a few minutes. But the closed cell plus whatever the thermarest contained within was enough insulation. (We were camping inside a tent, but on snow.) I used my summer 2 lb down bag, but also used another as a blanket over the top. I was toasty.
