most trunk rattle is coming from trunk lid, license plate, rear quarter panels, and the area around tail lights....if you deaden at least those areas it will help alot....now to be honest like a few people have mentioned you could also seal the box forward into the cabin of the car and lose nearly all of your rattle....it's well worth doing I have run a setup with 2 15s sealed forward and had ZERO trunk rattle...
like Crandis has his setup is a good example of what I am trying to say