Hand warmer into a small 6pack beer cooler bag ... you can find these insulated bags in supermarkets / camping shops ... Make sure bag ... batterys are all warm from inside house and close it up.
Put the hand warmer inside a glove or similar to prevent overheating battery.
Keep it closed unless actually taking battery out - and only take out when actually ready to plug in to AC ... close bag and put back in car ... or wherever out of the wind and cold.
If you had more time to order :
Turnigy Programmable Lipo Battery Warmer Bag (12v DC)
Runs of the ciggy socket in car or any 12v source ...
Personally I don't like the DJI way ... I'm one who values every mA in my batterys !!
Nigel