Rather then a damp towel hanging. I think you would see a better increase in RH and less work if you had a wick going into a bowl/bucket of water, and a fan blowing over it. Most humidifiers these days are wick based anyway so you can even get their specific wicks, though one for a torch would probably work the same. Anyway my last grow I was doing RH around 30%. It was so low to accommodate a very picky sativa I had, but I found much lower then that and I the plants got wind burn to easily.