Turns out the frost depth is only 5 inches, around here. I think 18-24inches of soil mixture should be enough to keep the snakes safe from the cold.

I feel much better about the winter months, knowing they're such hardy animals. Freezing to death is my main concern though! So thank you for all the information, Infernalis, I appreciate it.
@StephenBakir
That sounds like a pretty cool idea, could you elaborate some more? I couldn't use the idea, Im just curious.