You are limiting your ability to tune the vent using carpet as your resistive layer. You would get better results glassing a ring into the back with mesh (could use some Parts Express mesh grills of the appropriate size) ad fiberglass insulation (this is your resistive membrane) and another grill on the side that faces into the door or kick-making a sandwich. This way you can tune the membrane (mechanically) by adding/removing material and/or by compressing/decompressing material (resistive layer). By adding/removing material you are adjusting the impedance of the driver over a given set of frequencies. You will never get it perfectly flat, but you will be able to flatten it out quite a bit. I have posted tuning instructions for this on the board.