I also have a mesh top and had problems with keeping humidity up. I followed the directions in this tutorial and it worked like a charm:
http://www.ball-pythons.net/forums/s...ight=duct+tape
I will say that after the shed its been tough to get the humidity down. I keep enlarging the hole in the duct tape and its helping but I am finding I'll need to open a very large hole in order for the humidity to fall sufficiently.
I plan to use a sheet of plastic to insert in between the mesh and the hole to increase the humidity during the next shed.