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Aint we talking door panel heres?.

If you havnt noticed,
most vehicals, when door pads are modified to fit in split systems etc, mainly fibre glass is used to fit them in.
Beside MDF only, chipboard, plywood are brilliant enough to use as spacers.
Even sub, bass reflex boxes nowadays are made from perspec type material, which give those subs major oomph.

What ever you do, dont expect 6x9s on your door panels to sound like subs, they just wouldnt.
Wood, perspecs, fibre glass, plastic mainly help for better acoustics, mainly bass.
If yould like a little bass out of you Y sedan, I'd suggest, either putting in a sub in the boot, with a proper built bass-reflex box, other wise, just put in a pair of 6 or 8 inch sub woofers on the back board, preferably using chip board.

Having 6x9s on the door panels will just sound loud, which will give Tin sounding bass and not the deep hollow, humming type.

Anyone wanting good clear sound, without danaging much of the interior, I would like say,
-Go 6inch mids on the front door pads.
-a pair of tweeters mounted either at top corner of the windsreen or a either sides of the door posts, Just stick it on.
-A good set of 8 inch subs, 6x9s or limpids on the back board.
-And for bass, a set of subs in the boot, with a good spec built box.
Why I say SPec built box.
Any sub put in a good, specification built bass reflex box will sound good.
What I've noticed over the years, with myself entering sound comps etc, guys coming in with major 12, 15, 17 inch subs sound like $hit, cause of the boxes they built.
Having a bigger box, doesnt nessecarilly mean better bass