There are heat mats on the market that are designed to be used in the enclosure. I have several of the Intellitemp brand and they work fine for snakes that aren't too active, though I did have a retic destroy one playing with it:
https://www.bigappleherp.com/collect...tile-heat-mats
If you're looking for something more durable, Kane heat mats are designed for outdoor use in dog houses, or in barns for baby lambs, piglets, goats, etc. so they are very durable. They come in a wide variety of lengths and widths so they are very durable. You can find them on Amazon. The only thing I don't like about them is that they are solid/inflexible; they are rigid plastic.