Some people use plexiglass. You can have it cut about 2 inches shorter than your screen lid so that when you lay it on the screen, there will be about a 1 inch open space on each end for air circulation, and some people just drill a few holes in it for ventilation.
Or, if you like little projects, here is the link to a thread on a DIY cover for the screen:
http://ball-pythons.net/forums/showt...tment-Tutorial
Hope this helps.