When I first bought the brick I tossed the cardboard speakers and plugged in a cheap set of Kenwoods. They never fit quite right, as the holes in the speaker panel don’t align with the holes in the speaker. They were only able to attach to the door panel, as the screws didn’t match the holes in the sheet metal. I mounted them with only 2 screws each and had crappy sound out of the crappy deck.
When I got a new deck I went through the torturous process of rewiring all 4 speakers. I pulled the door panels out, sound-proofed with RAAMat and closed-cell foam and installed a wooden frame for the speaker.
I cut a square of thin wood to size, jigsawed an inner circle and used sheet metal screws to mount it to the inner skin of the door. The screws were long enough to mount it but short enough not to scratch the glass. The door panels fit pretty snug; they’re insulated with a thin layer of sound-proofing foam and the wood pushes them out a little more.
Once in place, I drilled the original speaker surround to deal with the speaker mount offset, then used wood screws to mount the speakers to the wood. They’re now super tight and the sound-proofing keeps everything from getting lost in the door cavities.