Ha this forum finally picked up on what I do!
Caulk all the edges of the trunk lid bracing cavities. I suggest windsheild glue but just basic caulk will work too
Blast expanding foam through the hollow cavities. 1 can has enough foam to do multiple lids.
Coat it with undercaoting
apply sounddeadener.
When doing expanding foam, make sure you cover EVERYTHING with plastic, it will expand and drip everywhere. Its a pain in the butt to get out once its on stuff.
Also dont blast foam where there are anywires, just incase you need to get in there sometime!