How many layers are you using? I use unprinted newsprint for the small boas, fold the edges up around all sides and then more in the center. If they get under, they usually get under the middle layers and not under the outside edge layers. I use a lot though, probably 15-20 layers or so. For my juvis and adults, I use singleface corrugated. Usually one layer but sometimes the boas will get under instead of using their hides. For the ones that do it often, I try to put 2 layers in but sometimes it just happens. And then of course, they poop under there, the brats. haha