Well, it's been over a year, so it's time to make good on my promise!
Well, the box housing the big green subwoofer would not fit into the small trunk opening in the Mustang, so that was out.
It's a convertible, so the back seats do not fold down, so no way to shoehorn that box in that way either.
Plus, there is a welded metal brace in the center of the rear of the trunk, pretty much making any attempt to get a big box in the trunk a futile effort!
What to do?
Well, I found a dual bandpass 8" powered sub by DUAL that was perfect for the job! And it slides in the trunk opening easily!