I had this same exact problem just a while ago w/ newspaper substrate and a tub set up. Here's what I did:
Your water bowl is your #1 main water source, which will somewhat keep the humidity up. I added another water source by taking a small dish, and placing a sopping wet sponge into it, then putting the dish into the middle of the terrarium. I also covered up some of the holes on top of the tank with tape, but left enough open to let the snake breathe.
Now, I only have to mist the tank maybe once a day, with humidity staying in the 70s for over 8 hours! Whenever I open up the tank, to replace some of the humidity lost, I mist the top of the snake's cold-side hide as well as the sponge in the dish. It works perfectly for me
Hope this helped!
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