Powered up the sub tonight. WOW! I'm not normally impressed with car audio speakers, but this Infinity Reference 860w sub is really accurate and has just enough bass with 400 watts running through it to shake things and rattle the license plate. I wasn't sure how the plastic box would hold up to the bass but I saw no indications that it's not structurally strong enough to handle the bass. It sounds very good also!! I have my amp cutoff turned to 80hz and everything down low sounds nice and smooth.
Anyways i'm really happy with the results. I can't recommend this project enough to anyone wanting to hear all the lows subs offer, and wanting to save space. The 6.5" box is perfect for most 8" subs - i'm just estimating here, but it looks like it's close to .35cu ft. I could be wrong but it's closer to what 8's need vs. the newer box they put into the '09-'10 Vibes.
Tips on converting the box:
-You will want to use a high strength adhesive on the ring(s) AND
screw them in from the backside of the box using wood or sheet metal screws and washers. The sub is heavy and you don't want it to become a missile in a car crash. So fasten it as strong as you can!
-You don't need to cut out the backside if
you choose a lower profile sub. The Infinity is one of the deepest 8" subs, but also one of the most highly rated, so that's why I chose it.
-Once you mount the ring on the box, you will have to seal 3 holes around the side. I used Bondo Glass but there might be better stuff (i.e, fiberglass, or maybe some kind of epoxy). Bondo Glass is known for cracking - only time will tell if it holds up. It's easy to chip off and re-do if that happens.