I was going to build a wood box, but thought I could never maximize the airspace that the factory plastic box has.
I measured the volume and I believe it was around .53~.55 cu ft so it was a perfect fit for an 8" woofer.
Cut the face of the box off, fit a piece of MDF to it, fiberglass'd it on, and bolted in the subwoofer.
Just did a quick and dirty job cause no one will ever see it.
I was so tempted to stuff a 10" woofer in there.. but it would have been choked.
Sounds great... hits harder than I ever thought an 8" would.
I highly recommend it!