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.
i'm leaning in that direction. thanks
my 06 gt had the shaker sub so I sorta felt I needed it. maybe the shelby interior 6 speaker + amp deal is all i need