i have built an MDF bracket covering half of the area behind the panel, its secured very good. yet i still got a very crappy bass responce because of the way the speaker is placed.
im using 7 inch woofers,completely soundproofed hatch in my integra and still not enough support for them i can feel the resonance from the frame and its destroying the bass from rear speakers
a set of lightweight speakers in the back turned out to be better than alpine type-r coax amped.
if you know how to fiberglass then i would suggest doing a complete fiberglass seal behind the interior panel.
otherwise putting 6x9 speakers in there is pointless, you not gonna get any bettersound if they have holes all around