Heating what? Snake? Lizard? Large or small? Ground-dwelling or tree dwelling? Tall cage or low height?
In a wood cage, I think trying to heat through the floor is a lousy idea unless the floor is plastic (make a cutout in the wood floor and put expanded PVC over it. Heat through the plastic.) And if you have a heat pad inside the cage, do not stick it to the wood floor. Have something like tile or plastic between it and the floor.
I prefer overhead heat in wood cages in most cases.
Big Apple Pet Supply sells heat pads that can be put inside the cage as long as there is no standing liquid. I encased a ZooMed pad in plastic and use that for a leopard gecko right inside the cage. Works great.
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Mark