It takes very high temps to ignite wood. The heat pads and heat tape sold for herp apps generally cannot get that hot. Leaving an airspace above or below the heater is usually enough to make sure temps are well within the safety range (for fires.)
What is more likley to cause fires is electrical arcs. This means that the integrity of the wiring and electrical connections is very important. This also means sure that rubbing boxes over Flexwatt for 5 years doesn't expose the conductors should the conductors come in contact with a ground.
So, the safest you can get is to use ready-made heat pads with molded plugs, leave an airspace, and for the safety of your herps, use a temperature controller.
If you use Flexwatt, crimp the wires correctly, cover the connectors with an insulator, and make sure plugs are attached reliably.
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Mark