I would second the consideration of insulating the battery from its cold surroundings as much as possible. An inexpensive used cooler from a yard sale might work well. Give the cooler a custom paint job and create a small opening at the highest point in the lid to vent the hydrogen. An inverted "P-trap" made out of 1/2" PVC would keep rain from running in the vent hole. If there is "extra" space in the insulated box, I'd add a few jugs mixed with an anti-freeze solution to trap more BTU in the space and buffer the temperature swings.
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