I've got a 20 gallon long terrarium from Petsmart, the type with the top screen locking lid, but I need to maintain a high humidity in there for my new critter (colombian rain boa) so I'm wondering if anyone knows where would be a better type of lid to get, same type of locking but maybe using glass or plastic? Thanks!
As was said, press-and-seal wrap. To hold humidity in my leopard tortoise's tank, I draped a plastic shower curtain over the top... But his is front-opening (and tall enough to mount lights inside--though that part is nonissue for a Rainbow Boa), so it doesn't get in the way. I imagine just draping would be pretty annoying with a top-opening enclosure, though.
You can take a layer of a similar plastic material, though (I like the shower curtain because it doesn't tear as easily as something like suran wrap) and tape a strip over the screen top. You'll need to punch a couple holes for ventilation, of course, but it's going to be your best bet to get high humidity in an aquarium set up... Especially since, last I checked, no one makes plastic or glass tops for Zilla's "Critter Cages" (the one Petsmart sells).
Otherwise, like AtlasStrike suggested, tubs hold humidity fantastically, or, another option that will probably be your best choice as he gets bigger, a professional plastics cage, like those manufactured my Animal Plastics or ConstrictorsNW.