Go with what the others have said here... unregulated heat lamps in that size of enclosure would be the death of your snake.
Personallly, I have a 30 gallon aquarium setup, and the ambient in my house & specifically my room goes down to 68 at night and remains between 70-75 in the day. Therefore, I'm required to opt for secondary heating to keep the ambient up. I have 2 rheostats, a precise zoomed one controlling my heat pad for the hotspot which keeps it around 88-90 degrees F. The other is a 6'' diameter, 150 watt(the bulb I use is 75 watts) infrared heat lamp controlled by a lamp dimmer positioned on the screen above the cool end, which keeps about half the tank's ambient at the right temps. The hotspot covers the rest.
The downside to using this is dry, arid conditions, though, so if you must, either place a larger water dish directly below the lamp or mist frequently. Normally, I do the former. During shed, I mist regularly.
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