Here is what I did this year, as it was especially cold in December here in So Cal, and my herps are in my garage = no insulation:

I put the snakes in a big tub (like species with like species), put some deep substrate, put a big strip of 11-inch heat tape under each tub on one end, set it to 58 deg, and let it go at that.

I noticed some of my rosies and kings sitting at the cold end with air temps at 48 deg, sometimes they were on the 58 deg.

This month, I started to increase the heat tape a few deg each day. I am now at 83 deg, and of course the air temps in So Cal at night are not as cold anymore. 6 of 8 rosies just fed eagerly and both kings did as well. All are good.

Anyway, point is, pretty simple and functional.