Yes, heat cable is safe for use inside enclosures with snakes. (I wouldn't try it inside an enclosure with something that digs, or chews, but snakes should be fine). It feels exactly like an electric blanket as far as warmth goes.
Thermometers, Thermostats, and Timers
I use the
Zoo Med 500R thermostat with probe It's not great (just has a temperature dial, not temperature settings, you have to dial it in by turning it on, waiting for it to cycle off, then check your temps. If too high, turn it down, if too low, turn it up, etc) but it was cheap , 33 bucks.
Threaded Mode