as for the PACKAGE TRAY (rear dash? haha) i made a custom one out of 1/2 ply, locked it down with carriage bolts, covered the metal in BXT, second skin spray and sealed remaining gaps great stuff non expanding foam....
if your not really trying to work, just use some deadener and spray little bits of the foam in there, should cure the rattles...just be careful not to foam over anything you might regret...