I keep and have bred BRB's. They need high humidity to survive, never mind thrive.

Instead of wasting money trying to make a fish tank with a CHE do what it physically can't (maintain 75% humidity at all times), get a tub with a securely locking lid like this:
http://www.lowes.com/pd_579877-62860...d=50150118&Ntt=

Move the under-tank heater from the tank to the tub (they don't go in the tub, always underneath it), and get a thermostat for the heater.

Cool side temp should be around 75*F, never under 70*F. Warm side should be 85-86*F, you can run the UTH up to 88*F (note that's UTH temp not ambient air temp).

If/when your snake ever outgrows that tub - and it will take years - you can get a proper PVC reptile enclosure for it.

Also BRB's are called "slender boas" for a reason. If you're used to handling ball pythons then they may seem too thin when they're really a correct weight.