I agree that real plants and soil are not recommended, bacteria, fungus, mold even bugs. For humidity I use a plastic sweaterbox (or larger for larger boas) with an inch of peat moss and an inch of green moss. Nice and damp. Need to check often for messes an/or mold. And I always change at least once every two weeks if not messed in. Typically gets messed before it gets a chance to get moldy.

You can see in my cage pic:

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