The two biggest factors determing the type of heat sources that can be used are the species that will be housed in the cage and the ambient temperatures of the area where the cages are kept.
The amount and placement of the ventilation also makes a difference.
If heat tape or other undertank heating is appropriate, my favorite design does not have a solid floor.
Instead, I just use silicone and staples to attach Sintra or another brand of expanded PVC directly to the bottom rim of the cage. Heat tape or cable can then be taped to the Sintra with foil tape.
This offers many advantages to a cage.
1) It makes them lighter.
2) It makes them more durable where reptile cages need it most - the floor.
3) It provides thermal mass to help distribute the heat from the tape into the air.
It also makes the cages easy to repair. Let's say you get a silicone failure and water gets between the Sintra and the wood. With the floorless design you can simply remove the Sintra, scrape off the silicone and re-apply it. If there were a small amount of damage to the wood you could trim it off. I once trimmed about 1/8" off a melamine cage before reattaching the floor.
This design also makes it easier to seal the bottom 2" of the cage inside with something durable. For snakes 2" clear packing tape works very, very well. Since snakes almost always deficate along the edge this is a good idea.
But some people just are not comfortable building a cage without a floor. I've never done it with pine so I can't say that it will work. I think it would, but I just don't know for sure.
In that case you could build the cage with a solid floor and then cut out a rectangle 2" longer and wider than the area you need to heat. Then silicone Sintra onto the inside of the cage and tape your heat source underneath.
This also works very well and is typically what people feel more comfortable with. Just understand that if water gets between the Sintra and wood there will be a lot more area exposed to damage and you won't be able to repair it in the same way.
And if you wanted to use the clear packing tape trick it would be harder to reapply with the floor still in place.
But ultimately if you take care when you first build the cage having to repair it should not be a frequent concern.
Regardless of whether you go with no floor or just a floor cutout, the three advantages I listed above still apply.
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Current snakes:
0.1 Gonyosoma oxycephala - (Silver/Yellow)
1.2 Gonyosoma oxycephala - (Green)
7.6 Gonyosoma janseni - (Black)
0.1 Gonyosoma janseni - (Black & Tan)