It looks to me like they're using some kinda stretchy poly...like maybe the stuff the stretchy pul is made from? And, I'd use netting, if you can find it. That way it's less likely to be punctured by something that wasn't supposed to be around it...like a pen or something silly like that. Just me. I think the stretchy poly would be cool, as it would allow for some "stuffing" of the animals.