Mazda3 Jack Subwoofer Enclosure
 

I was thinking about getting this enclosure from this website:
http://www.dibholdings.com/voxcaraud...3hatchback.php

I'm not sure if it's still active. I don't want to pay them if they're not gonna send me anything.. does anyone know if it's safe to purchase it from them? Thanks.

haven't heard of the website, but that seems like an ok price since it is somewhat difficult to make an enclosure like that on your own.
I don't believe that the dynamat really improves the bass response, unless the box is fimsy, in which case it would help stiffen it a tiny bit. As for the polyfill, use an old pillow (its guts) or get some of your own fill next time you're at walmart.
I'd think that will move a lot with no real strong mount for the sub in the rear end there - and that will diminish the bass response since every time the speaker pushes one way the box and everything will move the other way a little bit.