It is not impossible to maintain humidity in a tank. I keep my only BP in a 40 gallon (I think?). I regulate humidity by keeping his water bowl full and covering the screen lid. The screen is covered in several layers of tin foil and tape (NEVER put tape inside the enclosure, this is just on the outside of his screen). It was so effective that I don't have to mist (apart from when he's in shed).
You can also get a coconut-based substrate like repti-chip or eco earth. They hold humidity well.
It IS POSSIBLE to maintain temps and humidity in a tank, but it is harder sometimes, and if you are okay with spending the money, I'd get a better enclosure.