How to sound proof a car trunk.?

i have an 07 ford mustang with 2 12 inch subs in the back. stereo sounds great and all but on the outside it sounds like im dragging 2 trash cans behind me. is there any way to sound proof that trunk to get rid of the rattling sound it makes? i know about the dynamat product but i was hoping there was a product were i didnt have to spend as much money!