I have had pretty good luck with subthump. I have bought an amp rack and corner sub box for a 4th gen camaro, and a T-harness for my current 6th gen. They use pretty thick MDF and their stuff is well built. I think the guy who runs the site even had a 5th gen convertible himself, so he might might be able to give you some specifics on how that box fits.
-Geoff
-Geoff