yea...a old coffee can on top of a floor heat register in a house will add humidity to the air...set your watering container in front of the fan or air source feeding your grow closet..that should raise the humidity up ...any container in your closet with air moving across it should get you where you want to go....
no it wont cause mold if you change the paper towel everytime...i do this from time to time to raise the humidity if you take about five paper towel get em pretty wet and set them on the sides of the pot and the floor it can raise by like 20 at times once i do this but change that shit they dry really fast too
yea theyll dry up fast so just stay on it also you can get a ice cube tray fill the slot with a little sand very little sand and then fill it with water and set it n the room thats a long term solution