Yeah, the metal clips I use to hold down the screen tops on my snake tanks only cost a couple of bucks (literally) and the snakes definitely can't get out. When the corn snakes want me to take them out---especially if they see me near their tank---they'll go up to the top and start pushing at the lid with their noses; then when I unlatch the lid they come racing out, usually right to me. 
I keep Loki in one of those not-recommended glass tanks with a screen top too (inherited from the corn snakes as they grew into a bigger tank, originally bought at a rummage sale for $15)---but I cover the top with heavy clear vinyl, using a hole I cover or open to regulate humidity. I've never had a problem with the humidity or with the undertank heater except when I was recently gone for a week and Loki chose to stay in the damp moss hide on the cool side of the tank---which may have been too cool. He seems fine now, though. When he outgrows this tank, I plan to move him into a "real" snake tank, such as a Boaphile or Vision (which I'll also get for the blood python). Then I'll be able to stack the enclosures too, which would certainly save on floor space. 
>>We wouldn't worry about the crested - her enclosure is pretty secure, but the leopard gecko's terrarium just has a lift-off screen top, because that's enough to keep her safely in, so it's at least plausible to think that a snake could push the top up enough to get in. Not likely, perhaps, but possible. The conclusion we came to is that even though it might be construed as paranoid, we'd probably go the route of putting some sort of locking mechanism on the screen top in order to prevent any problems. Obviously, the caging I'm looking at for my potential BRB should be perfectly secure anyway, but with this sort of thing, there's probably no such thing as "too safe," and it should be a pretty minimal expense to get some clips to hold down the screen top.
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>>0.1 Albino Leopard Gecko - Tigger
>>0.1 Crested Gecko - Pooh-Bear
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Renee
1.0 BRB (Loki)
2.0 amel & anery corns (Foxfire & Daguerre)
0.1 blood python (Duchess, arriving later this month)
1.1 Cats (Nightshade & Cuzzy)