"But that isn't using the car as an 'ark'. Cars themselves aren't insulated, other than a fraction of an inch of foam attached to the headliner."

It still provides a wind break and shelter from falling rain and snow.
There is usually an extra layer of insulation above the headliner as well as insulation behind all the plasic panels conering the inside of the body.
You could pile snow up on three sides of the vehilce to help make insulation but I would leave one side open to get in and out.
Those reflective sun shades poeple put in their window on hot sunny days will reflect heat back in also.
You can also insulate your vehilce, I bought a roll of the reflective insulation from a hardware store that looks like bobble wrap with aluminum foil on the outside. They had a display where it was folded up into a littel pouch so that you stick your hand in and it refelcts your body heat and your hand gets warm. I removed all the plastic panels from inside my old truck and taped this covering everything and seldom needed the heater after that as it was always warm in the winter after a few minutes of reflecting my body heat.