We briefly tried using tubs but found our ambient room temperature was too low to make them work. Humidity was fine, but temps were way too low. So we switched back to tanks. Now that I have a dedicated reptile room that's much warmer, we are going to be transitioning our BPs over to a rack system.

I have seen some creative ways people heat tub ambients using heat lamps on stands, or if it's a taller tub, cutting out a hole and using wire mesh or hardware cloth on top of it to give the lamp somewhere safe to sit (I'd probably use a thermostat or dimmer in this situation).

If you do a Google image search of "snake tub with heat lamp" that may give you some ideas.