Ive dynamatted about 10 cars. Most of the cars ive done were Honda's actually. And all of them were done with like 2 - 12's running about 1000 watts RMS. So there was ALOT of rattling. Dynamat EXTREME is your best bet, honestly.
Theres alot about the sound dampeners. A. Alot of them contain like roofing material. B. Alot of them dont really work, have odors and sometimes stink up the car in the summer when it gets warm (especially because we have hatchs).
But heres what you should do. If you have a LARGE system, there are trunk kits, but your best bet it to actually go on eBay and check out DYNAMAT EXTREME BULK KITS. This is probably the cheapest thing you can get IF you do decide that you like the Dynamat brand.
From my experience, if you have a LARGE system, your going to need a good amount of dynamat, it really depends on how much money you want to spend, because putting any amount of dynamat extreme in your car will help. Dynamat extreme says that just 5% of dynamat in an area of your choice will already reduce dB's. Which is pretty good.
BUT, heres your second choice. Which I would SUGGEST to you.
Everyones seen pimp my ride. Well that really doesnt matter, SECOND SKIN AUDIO came up with probably THE BEST sound dampening products ive ever seen, reviewed, heard, played with and everything.
Their website: www.secondskinaudio.com
Now, they offer some great products, but one of the things im doing on my tC is the SPECTRUM sound dampening. It acts kinda like a paint, but honestly, this stuff is EASY to apply with a paint brush, you can pretty much get it into small creveses and heres its benefits: ORDORLESS! DRYS QUICKLY! SOUND DAMPENS MORE THAN THE MATTS! and much more.
By the way, did I mention only one gallon of it covers 40SQFT, which is more than enough to cover your entire trunk, and its only 50 dollars?
Seriously this stuff is the best, I would suggest you checking out the second skin products. There are way more benefits to using the SPECTRUM paint on sound dampening products than using the matts. Cutting those matts gets to be a pain.
The spectrum is what I will be dampening my car with soon. Also if you want REAL sound dampening, you can add their foam matts ontop of the spectrum stuff.
Also, they advertise that you only need about 2mm's or so of spectrum to notice a MANY dB difference, which is pretty good for the price.
Also, I dont want to let you in on my car (cuz when im done, itll be a show car).. But you can spray paint the spectrum, so you can have a blacked out trunk as well. You could even spray paint clearcoat over that spraypaint.. Now thats just I roll..
Hope this helps, if you have any questions, feel free to PM me or whatnot. I know alot about cars, audio, sound dampening, boxs, everything really. (maybe its because I work at tweeter center and install this stuff for a living)