I don't recommend putting your soundcard inside of a closed plastic bag... the generated warmth cant escape. I'd rather put a double folded bag on the backside (for 4 layers total) and optional another on the other side of the card, but i dont think bagging it up is a good idea.
But I have a Thermaltake Xaser III, it has superb ventilation.
Other people with normal/low airflow case might consider your option though.
Thanks for the touch