How are you heating it? If you are using a bulb, that will be your biggest problem. Also i'm guessing you are getting the Eco Earth nice and damp but not like mud.
Honestly you shouldnt really have a problem unless you have too much air flow. I use AP cages with aspen for my snakes and i mist once maybe twice a week and it stays 60-70%.
You can put in live plants. I have them in my GTP cage. There are lots of stuff you can use like Pothos, different palms, Prayer Plant, lots of ferns. Just stay away from Ivys as most are toxic. The only problem is a boa will most likely crush or tip over any living plants. My boas and retic and BP destroyed plastic plants so they all live in plain set ups now lol. The GTP is much more stealthy plus he is just a little tiny baby.
Maybe put up a pic of the set up and we can try and point out some stuff to fix it.