The thermal switch arrived and I tested it with a multimeter and the freezer. It appears to work as advertised (opens at freezing, closes above).
The switch is encapsulated and enclosed in a aluminum case with 8" leads.
I soldered the switch in series with the solar panel's positive lead and taped it's case to the battery.
When the battery temperature drops to freezing the switch opens and prevents further charging until it warms up.
The lithium battery is insulated from the case by a layer of foam.
?
N0QBH Mike Berg