My preferred way:
Cut a rectangle out of the floor that is about 2" wider and longer than the piece of heat tape that you'll need. Keep the cutout at least 2" away from the exterior cages walls/front as to not affect the structure and to say away from screws or other fasteners.
Route or chisel a groove from the cutout to the back or the side of the cage. This will be a recessed are for the heat tape cord.
Laminate a piece of Sintra or another brand of expanded PVC sign board over the entire floor. 1/8" should be fine, although if the cutout is excessively large you may need 1/4".
Using aluminum tape rated for heat tape, attach the heat tape to the underside of the expanded PVC in the cutout area.
This is not the cheapest way to use heat tape in a melamine cage, but I believe it is the safest and it also makes the cage floor more durable.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)