Running two bulbs in series or a dimmer are a good idea, a bulb rated at
1000hrs may last 5000hrs at 50% power and at 10% maybe 100000 hrs.
Thermostat heat tape may be better it lasts, comes on only when needed
and heats what is needed and the air. Insulate the box better with foam,
if it gets sun paint it flat black. Something flat black can reach
80-120 degrees greater than air temp, even at 0f. For a bulb to be 100%
efficient the box has to not absorb and loose any heat, probably a
mirrored interior. For heat tape you may only need 20-30 watts.