Several posts have discussed attaching plastic wrap or contact paper to the screen top to increase humidity. That works, but I figured out something that's even better. Lots of stores now sell thin, flexible, plastic cutting boards. Usually in a pack of 4 or 5 for about $5. You can use them instead of plastic wrap/contact paper. Just lay them on the screen top. You can move them around to increase/decrease the amount of coverage. That will adjust the humidity level. Has some effect on the temperature, too. The ones I have are almost exactly the depth of my screen top. One covers about 80% of the width, so I put on two and just need to move one to vary the amount of outside air getting in/inside air getting out.

Also, I got the Tropic Air humidifier (I live in a very dry climate) and it works like a charm. Humidity went from 40% up to about 80% in a very short time. (I know; I got it back down after fiddling around with the cutting boards).