09 malibu rear deck rattles.

I did do a search however most of the topics are several years old and not really like my problem.
As the topic states I have an 09 Chevy Malibu and the rear deck rattles a lot. If anyone here has ever replaced the 6x9s back there you'll know that between the sheet metal and the plastic rear deck there is a 3 or so inch gap. I've matted the hell out of the metal and even the plastic but still rattling. Would shoving foam rubber in between the deck and metal be a good way to resolve this or is there a manufacture that makes something specifically for this type of problem? I've looked at the usual companies and not really finding anything.
Since we're on the subject, does anyone make a deadener to go on the back of door panels, not the metal that's done, the actual plastic.
Of all the cars I've installed in this car has more vibration rattles than any other I've seen, must be all the plastic.

Thanks for you suggestions and help.

If you hold your hands on the top of the rear deck, does that reduce the rattle?  If so, then yes some kind of foam between the metal and the deck will help you. 

Yes that pretty much eliminates the rattle. I figured it would work just wanted to make sure there wasn't a better way. Thanks.