Cell phone warmer for storing phones in cold weather?

I'm looking for a way to keep cell phones warm enough to be functional in cold weather. The phones sit overnight in vehicles that are parked outdoors in northern Minnesota. The vehicles will be warm after 30 minutes or so, but I don't want the phones unusable for that long and also it might be hard on them to freeze and thaw every day. I thought there might be some thermos type thing out there that would keep them at like 40 degrees through the night. Any ideas?

Try and get a silicone cover for the device.

Silicone gets cold, but it won't freeze.

Tonight's low here is expected to be -16 degrees F, and that's just an average day in January. Really cold for us is like -30 or -40. I'm not sure a cover is going to help much. I've got a vacuum insulated food jar on order that I'm going to give a shot. I might throw some pennies into the bottom to help store heat. I'll report back with results just for the benefit of others living in the arctic tundra who may be wondering the same thing.

Update:

I put the phone inside a warm food warming thermos and set inside a vehicle parked outside overnight. Overnight lows were around -20 degrees F. Checked on the phone after about 16 hours and phone was frozen solid and shut off. Took several minutes in warm area for the phone to begin functioning again.

Back to the drawing board....