Best heating method for custom wood cages?

Hi, I'm building a bunch of custom wood cages, they're about the size of a 20 - 30 gallon tank and stacked on each other. I'm trying to decide how to heat them. I wanted to use something besides lamps but i can't use under the tank heaters with the wood. I've seen heat tape, heat rope, and heat pads that can go inside the tank on the wall but i'm not sure what's best or if some of them are safe to go inside of the tank. Any opinions on whats best or on ways to build the bottom of the tanks so they can use under the tank heaters or heat tape? Should i build them to share the same floor/roof like a shelf or make a space between them? Thanks.

I've used undertank heaters stuck to ceramic tile. I also stick the rubber spacers to the tile so the heat pad is raised slightly above the wood.

I've been doing this for years with no problems.

I've seen people still use belly heat beneath a substrate with no issue, but I've got a radiant heat panel in my wooden enclosure.

I'm building some wooden cages right now - seems like radiant panels are the safest way to go, though they are expensive as all %&*$^&@$!