If your using a screen top you might as well buy a whole room humidifier. Nothing you do will keep the humidity up in the tank untill you get that screen covered. Cheap way to do it is to tape tinfoil to the outside of the screen. Cover all of it right up to the light fixture. Those lights all have vent holes anyway, and if needed you can uncover a bit at a time until your humidity stays where you need it. Once this is done you'll probably only need to mist once or twice a week.