It's a no-brainer, but how you have to manage humidity depends a lot on the actual environment where you live. I could see that in New Mexico (which I believe is quite arid?) a humidifier could help. Here in GA, a ramekin of water in each tub is more than enough to keep the humidity over 60%; and each tub has two rows of ventilation holes on all 4 sides.

There are also misting systems targeted at arboreals. One mister can mist several enclosures. They usually come with a timer. It might be overkill for balls, but it's something else to consider.

A word of caution whichever you use, replace the water often so that you don't get a bacteria out break.