spray on a layer of ryno liner or its equal, i would think a continous layer would work much better, you would be able to get to tighter areas, and avoid seams (you would get with a sheet product) that could rattle themselves if they did not stick well (seen many come loose in our florida heat).
a layer of matting like NoTLaDStyle mentioned to fill voids to prevent panels from flexing, and to properly install your system with a well built box that does not rattle itself, firmly mounted to the vechile (3db gain if it is mounted and 10hp), not just laying in the trunk like 99% of the boxes out there.

 

i also use second skin damplifier. great stuff id buy it again.

 

We use Hush Mat at ny shop i like it better then Dynamat