I could joke around about putting machine screws thru the decklid and frame with some rubber washers...but I'm sure you wouldn't appreciate that too much.
Auto body shops have this black adhesive that mixes together as it travels down two twisting clear cylinders. It's like epoxy and dries hard like epoxy too, but it's black and kind of expensive, like $25 a tube I think, and the tube is just good for one use, it hardens and that's it for that tube. You may try MAPA Auto Body Store or any auto body jobber or even ask a body shop where they get that black epoxy like stuff. The tubes are about 12" long and look trick with internal clear twin winding small diameter cylinders.
I need to do that to my Pantera too, but if rattles bothered me when driving my Pantera, I'd be in a straight jacket, it makes a noise each time the frame twists, which is entering a driveway at an angle. My driveway.
If you find a internet link for the black epoxy stuff, can you post a link or make a referral please. That stuff seems to be what the original bodywork manufacturers use.