Look on youtube. There are videos of how guys have modified their Exo Terras with sheets of Acrylic with holes drilled in the top for ventilation.

I have a Zoo Med cube i use for my GTP and i have the top covered with tin foil and the screen hasnt rusted at all. It's been just over 4 months and i keep the humidity around 70-75% but i also dont have to ever mist as i use live plants and a big water bowl. I have heard though that the Exo Terra ones use a cheaper screen.