I have a screen lid as well, and just recently switched from repti-carpet, over to aspen - and after some trial and error have gotten my humidity issue fixed.
Here is what I did:
I took some run of the mill tin-foil, and measured it to the length of the screening on my lid - and made three pieces of the same size. I then taped the foil on the screen part so that the screen is completely covered by the foil. With a box cutter, I cut a couple of slits into the foil about the width of a pencil.
This seems to be working for me, and my humidity has been fine for the past day and a half so far. I also made sure to keep my water dish on the side of my UTH.
I also have blacked out with construction paper 3 sides of my tank, so that may be helping as well. :)'
I also second the opinion that a bulb is not needed. I don't use one either, and Kaa seems happy without it.