what I do for my screen tops with my BRB's is very cheep and effective.
Just pick up some of the thinnest countertop/cabinet surface wood (super thin fiber board with a smooth laminate on one side) which I think is 1/8 or 1/16 inch thick.
Then cut it to the size of the interior edge of the upper tank frame. This allows it to sit on top of the frame for the screen but be held in place on all sides by the tanks top frame. Make sure you have it laminate side down though.
In addition to eliminating almost all airflow it also, if you have cats, prevents them from popping out the screen if they get into the snake room.
You can also control ventilation by cutting a small hole in one if you need more ventilation after the lids are in use.
When I did this for my 60 gallons for my BRB's it only ran like $3 per lid.
Of coarse this does require you to have access to a table saw or other adequate cutting device for cutting thin fiberboard.
Hope this was helpfull,
Sean.