This might sound odd, but when I was into my installs years and years ago, I used to use the material they use within the fabrics industry. Not sure what the stuff is called, but it's like sheets of cotton wool style fabric that you can purchase in rolls. You can cut it to shape, it weighs nothing and you can stuff this gear into any nook and cranny.
Not too sure on the fire risks, but I used this material to stop plastics and my bumper at the time to stop rattling!