I have a screen top 40 gallon. I covered most the top with foil tape and mist daily with a mister. With that it was still low on humidity so I added moss to both hides. During a shed I keep it moist which makes the hides more humid than the rest of the tank. Since your snake stays in its hides most of the time it gets all the humidity it needs in the hides. Since starting this methods a couple years back my sheds are prefect with very little effort on my part.
The rest of the time I will mist the moss well and then let it dry out, then repeat.
Both big box pet stores will have moss. Is pretty cheap and can be cleaned so it last a pretty long time.