I would suggest attempting to cover as much of the boot as you possibly can. And depending on how much bass you want to have in your boot, would determine how many layers of the stuff you want to put in. In my car, the final plan is to have 2 X 12's and 2 X 10's, somehow fitting in the boot, so I have gone with 2-3 layers all round. Yes I know, may be a little bit of overkill, but the vibrations will stop and the sound quality will be much more improved. Also, if money is a problem, try searching for the stuff on E-Bay, or even google this product "B-QUIET". There is a website you can go to and buy it online, I just can't remember it off the top of my head. The products are much cheaper, buying it from the US, however it's the freight cost that will kill you.
Check that out and see how you go.