Here is a Q Logic enclosure for the 2009 mustang.
I emailed asking about the 2012 convertible.
I found a Ford / Blaupunkt 12" powered subwoofer tube for the 2010 Mustang
that comes complete w/ the plug & play harness. can be installed at the ford dealer. it's not cheap but it's not shaped anything special so I'm sure you an put it basically anywhere. Probably get a much better deal on a bazooka tube.
largest bazooka tube I could find to compare it to was a 250w 10" tube
for 1/2 the price.
heard back from shelby who told me to ask kicker. heard back from rockford who said they don't make them any more. haven't heard back from any of the rest yet.
during my weekly philly to nyc drive I really got a chance to hear the shaker 500 w/ the top down & volume up and I have to say it sounds great. those 8's in the doors really fill in plenty of lows. i'm very pleased & my sub seems like less of an emergency. nice to know I can take my time & find the perfect thing.