If you're hell bent on a sub, I'd have a shop fabricate a glass enclosure for the spare tire well. But the problem with subs in a convertible is that they have to be turned up so loud to be heard that they get obnoxious ... I know you mentioned being adult.
Instead, I'd probably look at the shelby kicker replacements for the 8" doors; or find shallow 8" replacements.
2012 GT Premium Convertible 401A Kona Blue Charcoal w/Grabber Blue 6spd manual Brembo Comfort HID Reverse Sensing Rearview Camera