Humidity has little to do with it - spraying the snake may cause stress and the idea is to eliminate stress.
I think paper substrate, whether it be paper towels, newspaper or butcher block paper is the best medium. I realize that some people like aesthetics, but the snake really doesn't care and a "whole" substrate like newspaper is safer and healthier than anything particle-based, which a snake can accidentally ingest or breathe.
In addition, for cleaning purposes, it's easy to remove all the paper and replace it. In other substrates it may not be apparent if, when, and where the snake has defecated.
That's not to say people haven't been successful keeping corns on a wide variety of substrates - I just think paper is best - especially with a sick snake.
Tim

Third Eye