I have not done it, but based on the room under the passenger seat in my '92 GT, it's going to be very difficult to do. Even if you could do it, I doubt it would give you the output you probably desire or would need in a convertible.
The best option for sound would be to eliminate the back seat and install a sub or two there. However, I realize this is not ideal for most. My only other recommendation would be to install one in the trunk. Again though, with a convertible, it may not provide the output you are searching for.
Sorry, I know that's not much help, but just my opinion.
Good luck.