Thanks for the concern, but I think I've got them pretty secure. I covered the sides and back with cardboard and the front crack is tiny. The only time it's escapable is when I'm pulling the drawers out, since the snakes can then go over the back plastic. But, then I'm right there and can get them before anything tragic happens.
That being said, they won't be used very long. The female has already almost outgrown it, and it's kind of a waste to keep a single snake in a three-drawer setup. My futon is coming to Walmart this Thursday, so I'll be getting separate tubs for them while I'm there.