Well keep in mind 90% of the time bigger enclosures create more problems rather than solving them. Smaller enclosures are easier to heat, keep humidity, help the bp feel secure, and clean. Also if your going to use a UTH to heat the tank you MUST MUST MUST have a thermostat or dimmer switch/rheostat to control it or you will end up burning your snake up.

Keeping humidity up is best done with a good moist substrate, cypress works the best and daily misting will help bring it up if it still drops low due to your house being dry.

Pictures of the enclosure would help, but i would avoid anything with mesh lid as that's just going to look moisture too easily, pick something preferably PVC/ABS/ or HDPE for an enclosure.