I don't know how much this will cost but....
First check out http://web.njit.edu/~cas1383/proj/main/ to get some info on fiberglassing.

Get a saw/cut out tool etc. and hack a space large enough so that the 6x9 will have enough clearance (it doesn't have to fit perfectly)
Build a MDF ring to hold the speaker (it should be larger than the hole you've cut so that it will stay on top)
Test fit the speaker. If everything is cool get something to bond the wood to the plastic of the interior panel. Use some body filler to smooth the lines of the MDF and the panel. For asthetics, try to keep it even on all sides.
****I haven't tried this myself, but others have said that filler WILL stick to plastic with ease****
Tape off all plastic areas of the panel and paint the new enclosure. Once dried, mount the speaker.

Keep in mind I haven't done this myself and can't guarantee it'll work, I'm just trying to think of ways to solve your problem. Do some reasearch into fiberglass to get some more info.