Haven't seen anybody mention a small fabric "tarp" type item as part of the kit for a shelter. In Artic survival school, we were taught to keep a piece of parachute fabric in the survival pack, to serve as a base layer over the shelter frame, then covered with insulation material. The fabric gave us a seal to the outside, with a decent layer of snow, we stayed outside at -40 and were reasonably comfortable.
I'd second the comment on the need for an insulation layer from the ground, even a simple polypad will add hours on to your comfort zone.
