My Commodore reacted well with self-adhesive foam strips. I undid my boot garnish (the plastic between the two tail-lights) and ran that foam all along it, then screwed it back together.
The foam is thin, and compresses well when you tighten the screws back up. I did the same for the licence plate... just take the plate off, flip it over to see the back of it, then run the foam strips along all four sides.
The boot.. my Berlina had reinforcing on the inside (same as the Skyline), and the welds holding the reinforcing onto the actual boot had cracked and come off with all the vibration. I stuffed a bit of foam here and there so that the two metals couldn't touch each other... dodge job here, and i'm sure a welder could have just done a few spot-welds that would have solved the problem.
The bumper, I dunno... maybe take it off, and check that the bumper supports (metal bracket things) aren't misaligned, then wrap the supports in foam, and hang the bumper back on them? Trial and error there, but maybe someone else has a better solution?