Given that the Model S uses much more energy in cold weather I have to assume that it's continuously heating the battery to keep it up to an acceptable temperature. But it would take less (a lot less?) energy to do so if there was some insulation between the battery and the cold air blowing underneath the car. A few mm of closed cell foam covered by a thin water-proof film ought to do the trick. Maybe just glue it on in fall and scrap it off again in spring?