I don't see a pic in your post, but I assume you are talking about the chimney? If so, like the rest said, no plastic, but what I used, as have others, is tin roofing. You can get used free or cheap if you can find it, or 1 new sheet isn't too pricey. Roll it up and screw/rivet into a tube. Yes galvanized tin can give off fumes if hot enough, but I have not found the tin to have even gotten warm to the touch. My box through the wall however does however get quite warm for the portion adjacent to the fire.