Here is a trick I learned from an old body shop friend of mine. Cut up a 2 liter pop bottle and use the plastic for anti-rattle material. And it is very easy to cut with kitchen shears.
Sounds hokey as hell, I know, but it works great!
The other problem with rubber is will trap moisture and accelerate rust. Plastic isn't as apt to do that. Plus you likely have some plastic 2 liter bottles or milk cartons laying around anyhow.
