Verne when I worked we done alot of welding, to keep the welding rods dry we would make a metal box, insulate, and put a light bulb in it. I would think using sheet foam you could do the same thing sense you use the generator for charging, you could try different wattage bulbs to get the temp you wanted, even have a vent so you could make adjustments if needed later. Just have enough size that the bulb doesn't touch the batteries or even have a simple to 1/2 chamber divider.
Steve Maxwell