Some of my snakes have "moss cushions" too, lol...as described above by Caitlin. And for a heavy-bodied snake like a BP, that's probably a better option than a completely enclosed humid hide, since fitting thru the "doorway" might be awkward for them. I do the same thing, replenishing the moisture from time to time, & my snakes will hide in or under the damp moss, usually peeking out & feeling very smug & secure. (it's pretty cute!)

You might need to cover nearly all of your screen top to maintain enough humidity for a BP. You'll notice that many endorse the professionally-made PVC/plastic enclosures because they "retain humidity better", but that has nothing to do with the material they're made of, and EVERYTHING to do with the fact that there are only a few tiny vents for air exchange. The more air going in & out of a heated enclosure, the faster you lose humidity.