The problem with that is that the paper towel will possibly grow mold and other nasties, and from the sounds of your description, you have no way to change it out for a new one. It would be much better to just use the foil on the screen lid. Then, when/if you need to up humidity, just toss a damp paper towel somewhere in the enclosure, and remove it when it is no longer necessary. A lot of people will put it under a hide, to make a temporary humid hide.