I'd concentrate on the floor of the trunk and the floor in front of the back seat simply because this is where the noise should be most prominent. I suppose you could try using an SPL meter to verify this is the case, someone probably has one they'd let you borrow for a couple days.
Materials you should be looking at should at the very least have a thin sheet of aluminum. Ideally you'd want lead bonded to the damping material. It's not as ridiculous as it sounds, thin sheets of lead are flexible and not THAT heavy (~1 pound per sq foot). McMaster-Carr sells them for sound BARRIER purposes. After all, it's much more effective to not let the sound in in the first place than it is to damp it once it gets into the car.
Constraining it to damping material will be even more effective. Sound must travel through the damping material, then gets reflected by the lead sheet back into the damping material again.
If you go with an off the shelf product, try to get one with as thick of a barrier layer as possible. Think reflection rather than absorbtion.