I've done a little bit of searching and got some mixed results, some places say the fiberglass looks better but the audio quality suffers, other places say that as long as you do it right, ie make it think enough, make sure its sealed good, ect. that audio quality is just as good as MDF. i'm wanting to cut the centers out of the back panels and built a fiberglass box into them, if this is the general shape of our trunk
 Box_idea_1.jpg ( 25.24K ) Number of downloads: 2

then this is kinda what i wanna try and make
 Box_idea_2.jpg ( 27.69K ) Number of downloads: 2

i'm just not sure if building fibaerglass boxes into the existing pannels will sound ok as long as i do it right.
also how does the shape inside the box affect the sound? does it matter wether i make it square or more of a rectangle as long as i have the right internal volume for the sub?

Answering things backwards, as long as the internal volume is correct, you'll be fine I think. The hardest part is making sure that the sub will fit in place; don't forget to take a rough estimate of the space the sub takes up out of your internal volume.

In so far as fiberglass vs MDF, it depends a bit on what you are trying to achieve. The box in my avatar is a 3/4in MDF sealed box with a pair of twelves. Its tight, precise, small, and sounds great. If you want that big BOOM from subs, you want a ported or even bandpass box. Again, MDF works great. Fiberglassing is probably better for the former: its easy to make sealed and precise.

I'm sure SC-Customs will see this thread and comment on it; he'll know better than me about this stuff...