All plastics. Consider that the "bins" aka "tubs" that so many use were never designed to hold living animals...they're designed for storage, not for heating or housing, and certainly not for constant heating. Just saying...I know, in our modern world we use plastic everywhere, & those of you who are younger have never known a world without it. But ask any chemist about the toxicity of plastics, & remember that you can smell those plastic bins for a long time even when only used for room temperature storage of sweaters & such. Now consider that you're heating the plastic to house snakes: if you never took chemistry, the thing to know is that heat makes the molecules more active & more likely to off-gas, where your enclosed pets are forced to breathe it 24/7. The self-preservationist in me wants to minimize my own exposure to unhealthy stuff whenever I can do so, & I feel the same way about my pets. Cancer & other diseases may take a while to develop, but the more exposure you have to things that are unhealthy, the greater the chance you have of causing genetic mutations that lead to serious illness, & presumably the same for our pets. None of us can avoid ALL bad things in our environment...it's impossible these days, but that doesn't mean we shouldn't do the best we can, does it? (yes, I'm a "label-reader" too, lol) Food for thought.