Bryan,
Heat cable/rope is already sealed, much more flexible, and uses the same technology as the heat cable designed to be imbedded in mortar under bathroom floors.
Much safter choice than flexwatt, IMO.
The air space around the heating element is not a big deal if the material covering it is fairly conductive. Plaster, concrete and tile grout are all conductive so this should be fine.
Remember, the recent discussions regarding air space have had two different themes. The air space around the heating element and the air space in front of reflective insulation.
Not sure if you're confusing the two.
But back to the point of your post, the safest choice for you is to use a radiant heat source directed onto the fake rocks. It's better and probably preferred by the animals as well.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)