I use a mix of eco earth and bark, its fantastic for keeping humidity up. I literally put it in very damp, and every two weeks when they're out feeding, I pour in water, mix it all up again and they're all set. If it drys out a little too much through the week, I mist every now and then, but I find its pretty low maintenance to be honest. It last about 4-6 weeks before it needs changing, but it depends on how often you're feeding and said snake is pooping/weeing! I'd say either fine or coarse would do the trick. I buy it compressed in a "brick" and then a bag of the bark to go with it.
iv just switched to eco-earth for my brb and its doing the job nicely. I was using the lucky reptile bark with added moss but then i found packs of eco-earth, 3 blocks for under a fiver, which works out so cheap, so made the switch.