I decided to try to make my car quieter. If you notice in the hatch back there is an access panel to replace the light bulbs on the marker lights. This gives you access to the hollow back fenders. These hollow fenders amplify road noise like a drum. I decided to fill this area with expanding insulation foam. I removed the plastic panels then I took a paper cup, cut the bottom out. This will make a tunnel to the marker bulb, so the foam would not prevent access. I has been some years since I bought this foam. It does not expand to fill the void like it use to. One can would expand 2-3 times its size over a few minutes. I was unable to find any expanding foam. The stuff now stays the same size that you spray out. It would have taken 5 or 6 cans to do the job each side. So I filled the area with a combination packing peanuts and the foam to form a solid fill. Now my car is much quieter. I hope this works for you.
Bill

I've seen that before .....and it causes rust through quickly.

Just my 2 cents and recommendation . I would remove the panels and then take out that expanding foam before it gets wet and starts rusting. That is a great way to quite the car and then later get rear quarters to replace your rusted ones. Seen it on cars I looked at before, and it rusts it badly.

There are some inexpensive mats you can get (other than dyno mat or something).