The DynaMat helps to a certain extent but it's not perfect. I used to have two Kicker Competition 12"s in a sealed box in a Honda Accord. I laid DynaMat all over the inside of the trunklid by cutting pieces with a utility knife and sticking it on the sheet metal. It didn't make a huge difference, though. Part of the problem with the Honda is the trunk is made from thin metal so it vibrates a lot to begin with. If you guys are trying to cure rattles in cars like that (as opposed to heavier cars like an older Impala, etc.) I personally would save my money and just imagine a gorilla's tryna get up out the trunk.
The other material that people mentioned is the foam/sealant material, but you have to be careful. After you apply it, it expands, and it's also semi-permanent, so make sure you know what you're doing. To my knowledge it does not serve the same function as dynamat. The foam is made to secure your subwoofer enclosure in place in the trunk so it doesn't move around. I have seen & heard this used in free-air enclosures, where my homie had two 15"s mounted to a board, and then sealed the board near the backseat end of the trunk in his Regal. The sealent/foam secured the board and prevented it from rattling back and forth or moving when the subs hit.