I had a couple 12W0s in a sealed box facing straight upward in my conv. Mustang. It sounded awesome for what it was.
I chose a sealed box because I could barely squeeze 2 cu. ft. of box in the little ass trunk. I also like the crispness a sealed enclosure produces, so it worked out great for me.
All depends on what type of sound YOU want and how much space you have to work with.
For very tight, controlled response a sealed box (smallest) is reccomended.
If your'e looking for more boomy response with more spl, I suggest a ported (larger) or bandpass box (largest).
Whichever box you chose be absolutely positive it has the right airspace and is compatible with your sub.
Either way you go, with a convertible with limited space try to find a low Q sub that will work well in a small enclosure.