Throw some cypress mulch in there if you really want to jack up the humidity. I'd go with a 28Q tub for now, maybe even 41 for her. I'm warning you right now, you're going to want another boa, very soon.
Opinions vary, of course, but unless she gets larger than a typical boa, a 4X2 PVC cage will be best for her. They are lightweight, easy to clean, and hold heat and humidity very well. I prefer Animal Plastics, but boaphiles are highly regarded also. Another brand is Boa Master. They look nice, but I've never used them or known anyone else who has. Best of luck!