Warming Batteries

Are you getting the power for the light, from the battery ?
Is the battery fully charged now ?
Will the battery be on a FLOAT Voltage charger ?
Is the battery unattended during the winter ?

A fully charged battery should be OK down to -70 F or even lower.
The battery manufacturer should provide minimum temperature vs SOC specs
Your outdoor temp is -40 F / C ?
I assume you have Solar Panels to maintain a 100% State of Charge via Float Charge ...
So technically, Fully Charged the battery should be OK.

If you have excess power from the Solar Panels then I would use a "12v Heating Pad & Thermostat" vs a light bulb.
Insulate (but do not seal) the battery box.
Use an adjustable bulb-type freezer thermostat to control a raw heating pad, something like this one ...
http://www.ebay.com/itm/100-Watt-12-V-Volt-Silicone-Pad-Heater-SVO-WVO-Vegetable-Oil-Diesel-Fuel-Tank-/192062771380
Do not buy a heating pad that comes with a controller that has the 30 minute auto-shutoff safety feature.
Power the Heating Pad from the Solar Panels only, not from the battery.
Put the bulb/probe of the thermostat inside the battery box, keep the switching unit outside.
The lead is massive and will hold heat during the night.

Maybe, you could keep the internal temperature of the battery near 0° C in the box when -40° C outside?

Only, a Fully Charged battery is safe down to -56° C.
A fully discharged battery will freeze at about 0° C.
So, keeping the battery at 100% SOC is very important.
That is my reasoning for not using any power from the battery power for heating.