I use a human heating pad for my quarantine setup (it's a sterlite tub and the pad is kept on the outside). You'd have to put a thermostat on it to regulate the temps. However, I have to agree with the other post about trying a radiant heat panel. If you go with a light be sure it's outside the cage and be sure your temp measurements are correct - use a digital with probe thermometer and/or temp gun and measure underneath the mulch. Also be sure the light doesn't mess up your humidity (60% during nonshed and 70% in shed are great numbers).
I realize you do find some online caresheets that say lights are okay, but the general consensus is belly heat is better if possible. Be careful what you read and take as good tips on those caresheets. Often times they are simply regurgitated outdated information - sometimes they're great resources.
I'd love to see pics of your snakes if you have any. 

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