Wish you were local to me....
Of course herein lies the dilema. If our definition of loud is going to be purely an excercise of generating the highest intensity standing wave at the windsheild of a car (which really it would almost have to be since that's easily quantified), we come to the factor of vehicle accoustics which IMO is probably worth more than the type of equipment anyway. My jeep that's falling apart with cracks in the panels vs. your CRX with 5 layers of deadening and tight gaskets, or cement filled doors...... not saying that's what you have, but just illustrating a point with extreme example for the sake of illustration.
And really I think you would agree that this will be pimarily an excercise of who has more experience designing loud enclosures which I'd give to you hands down without a challenge.
All that being said, this still sounds like fun, what vehicle would you be using and how much reinforcement is in it?