Sorry, I misread your post as why do people.... Lol.
I used some sticky backed foil covered dense foam rubber from eBay, works quite well. It comes in large sheets or a roll which you need to cut to size and shape.
You don't want to make it too quiet cos you won't be able to hear things fall off, also, now mine is quieter, the missus talks to me whilst I'm driving it, doh.