Here's what I'm thinking: If there are enough people who have interest in this, I'll go ahead and do a little more research into cost and materials. After I figure out the cost I'll see who wants one and start building. The three designs I have are a wedge which would sit in the back of the trunk and not take up much space, a fiberglass box that would go in the wheel arch, which I believe is where the RF sub goes, and another fiberglass box in the spare tire well.
Here are the designs I came up with at work this morning:
The wedge would be simplest, and the cheapest. All MDF with rounded inside edges.
The wheel arch would be close behind and be easiest to ship since it will be small and all fiberglass
The spare tire well would obviously be HUGE and hard to ship, but if somebody really wanted one, I could do it. It would be the most expensive as well since it's a ton of fiberglass.
My goal is to make quality, affordable, and strong boxes that fit our cars. A lot of stealth boxes online go for 300+ and my goal is to do it for less than half that price. So who's interested?
NOTE: the drawings are poor and not to scale, so please don't rag on the sizes being unrealistic. Thanks!
On the wedge box you might be able to mount the box between the subs. 
