I too have a cabrio with rear speaker needs. I have not done this yet-but I will. If you use marine speakers-you get water tite enclosures that are made to mount to tubes-like your rollbar. No loss of space-great sound-water tite. They are a bit more costly than standard speakers. I've added links in a good-better-best format.
Good-http://www.crutchfield.com/p_206KMT6/Kicker-KMT6.html?showAll=Y&tp=3384
Better-http://www.crutchfield.com/p_1086000M/Infinity-6000M.html?showAll=Y&tp=3384
Best-http://www.crutchfield.com/p_1086100M/Infinity-6100M.html?showAll=Y&tp=3384