Just did the poly-fill in the hatch and oh man! This is definitely one of the best kept secrets out there.
The hatch buttons pop out, it just took some playing around with a flat-head screwdriver. If you're having trouble, put the flat-head underneath one side of the clip and move left and right until you hear a click or two (that's the clip coming out a little) then you should be able to pull it out.
I stuffed the crap out of it with poly-fill and now it feels like a real hatch door (feels more like an older SUV hatch, nice and heavy) Although be aware that if you do this correctly, the hatch door will not open all the way by itself with the hydraulics (not a big deal in my opinion but i know a lot of people who wouldn't like it)
Poly-fill can be purchased at Wal-marts for around $3-$4 a bag. I only used 4 bags for the hatch.
While I had it open, I also tightened the license plate light bulb/hatch handle bar (I think EVERYONE Should do this!! For whatever reason, after driving the car for years it becomes loose)
Then I tightened the screws on the license plate and used velcro for the bottom of the plate (instead of drilling new screw holes).
Not only has ALL of the rattling in the rear area of the car gone away, but my 12'' Sub punches so much harder and cleaner with everything tightened up!!
(NOTE: If you want the cleanest possible sub sound... point the sub up toward the roof of the car and lay it in the exact center of the trunk-area. This allows a clean pumping as it does not get buried into the hatch or the seats.
Enjoy your insulated rides
Next on the list: Poly-fill all the doors so that when people slam them it doesn't sound like an aluminum can being stepped on! hahah