They actually don't require belly heat, that's just one type of heat source. Any heat source can be used, as long as the area or "hot spot" inside the hide is at the appropriate temps between 87~91.
And of course, as long as humidity can be maintained above 50%.
well I worded it a little wrong, I “feel” like the best heat is from a uth with a thermostat, then on top of that in my enclosure I have a heat lamp. But that’s just to keep the ambient temps good as I keep my house at 68*
In a tank? Cover the screen top with foil/plexiglass (there's a tutorial on here somewhere) or a damp towel, use a coconut-husk-based substrate, bigger water bowl, moving the water bowl to the hot side/ under the heat source, using a humidifier in the room where your snake is kept. That's all I can think of off the top of my head.
Some people get reptile misters/foggers. They're not particularly recommended for BPs, more for high-humidity amphibians and lizards such as dart frogs, but I've heard they can work as long as you put them on a timer to regulate how much they put out? Thing is you don't want the enclosure to be wet, just humid.