Cold weather options for LifePO4 batteries

This thread wasn't started to debate the merits of LifePO4 vs other technologies. For various reasons I elected to go with it, very happy with performance thus far. One variable that has suddenly become more of a potential issue is cold weather charging. We purchased a plot of land up north and are camping there, and we've seen already some nights down below 0C. We are completely off grid, with solar and generator (only when needed) as power sources. For the couple of mornings where we had frost, I ran a space heater in the front compartment to get the batteries up to a temperature where I felt it was safe to charge (the solar comes on progressively through the morning, starting with a few amps and then ramping up to 80-90A by mid-morning). With the battery capacity I have (600AH, probably 500 usable), using the space heater isn't a problem, but it's not very efficient, and it's not a solution for when we're not there (we go up for long weekends right now).
I will be adding more insulation to the compartment (already have some R12 styrofoam sheeting, but will make a box for the batteries). I also ordered a small 12V PTC fan heater that will draw maybe 12A that I ordered online that I would install in the compartment to run full time (or maybe on a temperature control?).
Looking for other suggestions, I've seen recommendations for use of small 7.5 watt elbow pipe heating mats. What are you using if you are in the same situation?
Thanks in advance.