I have had 12 years in the plastic film industry and I can assure you that most plastics (PVC included), will begin to release dioxins at around 150 degrees. Dioxin has been implicated in the onset of breast cancer as well as other illnesses. Please folks, keep the plastic away from heat. It's like waving around a loaded gun. It may never go off, but I sure don't want it pointed my way if it does.
For the vent mod, I used roof flashing. Be sure to use the stainless type and not galvanized. Just measure the diameter of your stack and cut the flashing a little longer. Roll it into a cylinder and insert it into the stack. You can then adjust it up and down to allow the grills to be removed. Remember to wear gloves when you play with it though. That stuff will bite you!