I use a similair technique to the foil. I covered the screen on my 40g glass tank with clear plastic vinyl from Wal-Mart sewing dept.. It is on big rolls and they will cut it for you.

Covered 80% of the screen and can slide if I want to open it more. That and a big water bowl do it great. During shed I spray the Aspen bedding to bring up the % to 70 and it holds humidity like a champ. No more maintenance than replacing water every couple of days with fresh.