Ok, in response to requests I am posting my Screen Foil Treatment step-by-step picture tutorial here in the caging forum. :) I hope it proves useful.
SCREEN TOP TREATMENT PROCESS
Step 1 - Supplies. Screen top, aluminum foil, Duct Tape, box cutter or Xacto knife, and clear vinyl (optional. Wouldn't show up in pictures)
http://i184.photobucket.com/albums/x...s/Supplies.jpg
Step 2 - Tear 3 layers of foil, sized basically to your screen. (had to use two sets of three layers for this square top) Lay them shiny side down.
http://i184.photobucket.com/albums/x...cts/Foilon.jpg
Step 3 - Tape down the edges with Duct tape.
http://i184.photobucket.com/albums/x.../Tapeedges.jpg
Step 4 - Continue with strips of tape to cover the top completely.
http://i184.photobucket.com/albums/x...ts/Covered.jpg
Step 5 - Cut the ventilation opening on one end of the top with the box cutter or Xacto knife. Be VERY careful not to cut into the screen! The opening should cover about 1/4 of the top - typically 5-6" square.
http://i184.photobucket.com/albums/x...Cutopening.jpg
Step 6 - Tape the edges of the opening to secure the foil. (Indentations mark the edges of the hole)
http://i184.photobucket.com/albums/x.../Holeedges.jpg
Step 7 - (Optional) Cut the window opening near the center of the top. This lets light in without sacrificing humidity.
http://i184.photobucket.com/albums/x.../Cutwindow.jpg
Step 8 - Use the cutout piece of foil/tape from the window as a template to cut vinyl, adding 1/2-1" extra to edges.
(NOT SHOWN - I pre-cut my windows, and vinyl doesn't show up in pictures.)
Step 9 - Lay the vinyl UNDER the foil inside the opening (against the screen) relatively centered.
(NOT SHOWN - camera wouldn't pick up the plastic)
Step 10 - Tape the edges of the window, similar to taping the ventilation opening edges.
http://i184.photobucket.com/albums/x...indowedges.jpg
The Finished product! This screen top was to an 18x18 ExoTerra tank.
http://i184.photobucket.com/albums/x...s/Finished.jpg
To show what a more usual screen top looks like treated, here is my 20gal Long locking top:
http://i184.photobucket.com/albums/x...kscreentop.jpg
And my 10g locking top for contrast. The window of this one was larger to let in more light in a dark room:
http://i184.photobucket.com/albums/x...gscreentop.jpg
This is the most effective screen top treatment for regulating humidity in glass tanks! Second would be plexiglass, but that can be pricey. :)