I posted this on Spyderchat, thought people here would like it too. LTO batteries are now pretty widely available and can be safely charged at -20C, so I think it's a good solution for folks in cold weather areas.
Here's a pretty economical one:
https://www.amazon.com/MiSolar-Energ.../dp/B078RWMGN3
(says sold out right now but the price was about 180 last I checked)
It's a little pricey at over 360 if you want to use 2 to get a more worry-free 17Ah capacity, but it would essentially last forever, so you could even move it to the next car to use.