just a note of philosophy echoing what's already been said. Good suggestions have been offered regarding protecting the bag from embers. I would say, though, that I accept the occasional ember if I use a fire, and though it is possible to experience a bag-ruining (big big) ember, for the most part I just don't worry that I might have to patch up an ember hole or two in my expensive down bag.
Sort of like worrying to death about the first scratch on a new car. We eventually get over it.