Help with Grand Prix floating rear speaker deck

I need a suggestion to solve a problem with my rear deck speaker install in my 99 Grand Prix GT.

I replaced the stock 6x9s in the rear deck, but the deck that the speakers mount to is the lower, metal deck, and a carpeted deck that contains the molded speaker grilles floats above, only making direct contact with the seatback and rear glass. The speakers sound horrible, booming everything but the highs, I think because of a gap between speaker and grille.  The space between the decks is 3 to 4 inches high and the width of the entire deck. The gap between speaker and grille is probably less than an inch. I can press the grille and the deck flexs up and down, but doesn't quite make contact with the speakers.

I thought about re-mounting the 6x9s directly to the top deck, but it's made of flimsy cardboard. The new speakers are using the stock 1" spacer, but I'm unsure how much more space is to go to meet the upper deck, as it's impossible to see in between the decks, and they are a bear to get on and off.

Any and all ideas are welcomed!

I got it figured out. I picked up some 3/4" closed-cell foam for insulating around air conditioners from Home Depot, and stuck it around each speaker and also under the edge of the grilles. The two pieces meet and seal off the speaker when the deck is put back in place. No more Bose-style muffled booming!!