I stumbled on some 1/2” commercial carpet pad (like heavy gray felt) and cut it to fit. I just kind of friction fit it back there. BIG differenceYes. Also keeping a lot of stuffing in there cut the 'rumble'.
I use 8 packs of paper towels as filler.
Two of those goes a long way to stopping the typical hatchback rumble.
Seats down makes a difference too, try that with no cargo cover for full effectI brought a 58" 4K TV home the other day and had to leave my hatch open, resting on the TV box.
Let me say the exhaust sounded glorious with the hatch open !
i've never drove with my cargo cover out though, going to give it a whirl!