I also considered good old fashioned light bulbs, that generate heat. They say to heat a doghouse in Calgary with a incadescent 100W light bulb, to keep the dog warm, not sure how low of a temperature that would be effective.
That is a good idea...
To keep warm the project box, the use a Heat lamp control by a LM335 <-- inside temp monitor and of course insulation <-- The "pink" stuff, and make sure the inclosure is seal. ( lots of wet snows/freezing rains/rains/snow )
Just an idea...