Beaky..
I.m on the west coast of British Columbia, Canada...you know (igloos, fur trappers, etc.) I agree whole heartedly with
NOT leaving your gear in a vehicle. Never mind the cold aspect, but I doubt your "average" instrument insurance would cover it from theft, if your vehicle was stolen.
Getting back to the cold thing, I have plush lined,zippered, soft shell cases for the 2 kb's that I use, but I also have 2 reflective survival blankets, that I wrap them in before putting them in the gig bags. This of course is during the winter months. In Canada that's often 13 months, depending where you live.
These blankets are basically a very thin tarp, very light-weight, with a silver reflective side, and you'd be amazed how they will keep a unit warm. Not dangerously warm, but just at a nice even temperature, at least while they're being transported. You can get them from any outdoors or camping supply store
Works for me....L
Good Luck