Agreed, that if your plastic is melting, your snake has worse problems than fumes. Nevertheless, the thing is...
When I brought home two new tubs from the Dollar Store, they stank. I hate that smell! Felt guilty putting the little Kings in those, but it was the best I had. They didn't seem the worse for it, but as soon as I could, they went into glass tanks.
Ambient temps aren't such an issue here. Thick adobe walls maintain a steady temperature, and we heat with wood. Added humidity is easily maintained at any level by covering the top.