I saw a post a few days ago about how you can hook up a humidifier to a vaccuum hose and attach it to the screen so that it pumps into the enclosure. Or if you can't figure that out you could just hook one up the room that your snake is in a boost the humidity of the room and not just the enclosure.
What type of heat source are you using? A heat lamp and a big tank are a pretty bad combo, they suck the humidity right out. I couldn't keep it up in my 55 gal no matter what I tried, but I had to keep the lamp because I couldn't keep the ambient temps up. So in the end I switched to tubs.