I have the same set up and have for a few years. Easy to put together and just gets jammed in a corner when no needed. The product give s a temp at 180deg. THis is when you will begin to have off gassing fro the plastic, but may be much lower than ignition. I find it to be an acceptable level of risk especially since it will be on the outside of the smoker. On most models, the main air intake is at the bottom, which I did not insulate. Really not different than microwaving your food i a plastic bag if that calms some fears. After a few higher temp cooks, the material stiffened a bit, but overall performance really wasn't impacted. I'm going to use it tomorrow again during a -9deg day....should be fun.
I would recommend this if you don't have the cash to weld a second wall on your upright and fill with rockwool. If I'm going to all that trouble, I'm just getting a better smoker.