yeah, damplifier is some good stuff and the owner is a good man. raammat is also very good, and also cheaper than damplifier. i chose to get damplifier and i loved it.
also, when youre sound deadening, you should just go all the way and do the whole trunk. you'll love the outcome and its a fun project. will it stop the rattling? maybe, maybe not. it has the potential to. first you should listen around your trunk to isolate the rattling and see if you can stop it with foam or something. but yeah, deadening will help, how much depends on what is actually rattling.