The 'Foil on the screen' method works quite well for me out here in dry Colorado, but since I also use low watt lamps for ambient warmth and visibility, I top my screens a bit differently:
I still use three layers of aluminum foil, shiny side down to the tank, secured on top of the outside of the screen with a layer of duct tape. I leave a square hole (6"x6" for a 20L tank top) over the warm end for ventilation, but I also cut out the center foil in a rectangle and place a piece of clear vinyl one inch larger on all sides under the foil layer and tape in place on top. This allows light through the top but still holds in humidity and temperatures.
10 gallon tank slide/lock top:
20L ZooMed screen top:
As seen in the 20L picture above, for boosts to humidity you can place a dampened wash cloth (or paper towel) over part of the opening in the foil.
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