Papertowels are fine. They do dry out quickly though. You could try to switch to Cyprus or Aspen shavings to help keep humidity. To help keep humidity in, cover the rest of the screen with foil tape, plexiglas, or even a dry towel. I'm not a fan of the wet towel thing. When I tried it back when I was using tanks, all it did was make it humid in the room and not the tank. Considering balls are at the floor of the tank, and hot humid air will rise I never found it helped my ball. I'm a spraying person. I mist the tubs 1x daily and that does it for the whole day. If you cover the screen and mist I think you'll have a less hard time keeping humidity.

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