Will be doing this to my 06' convertible 'Stang.
Will be making a smaller box due to the roof box stashed in the trunk...
I'll be maxing out the space with 2 10" Kenwood subs @ a Kenwood amp. (As of 9/27/12 $134) @ amazon: Amazon.com: Kenwood P-W1010 350-Watts 10-Inch Bass Party Pack: Car Electronics
My design allows the spare cover to be lifted when necessary. My cover is beefed up with 3/4 plywood floor so I can carry lots of stuff.
The speakers will face rearwards, into the trunk area. Does that make much difference? Does the phase need to be reversed?