I'm not the biggest experts on BP here (I only have the two), but I know that with the BP it's important to have the entire thing be humidified, not just the substrate. (and how neat that you keep tarantulas! If I get another critter it would be that!). A lot of people keep their BP in more closed containers especially in dry climates. Also the probe should probably be about halfway up the side of the enclosure.
I have an extoterra enclosure for each of the snakes I own. I have the top covered but not airtight, and those terrariums also have small airholes right below the swing doors. That is enough ventilation for them without letting out all the humidity.