I dont think that anyone has a pre-made enclosure for a 'vert. Check around your local audio shops about building you one. Preferably a JL dealer as they would know what size box to build. You may be suprised, the cost of the box may be pretty close to what the premade (i.e. Stealthbox from JL, Thunderforms from MTX) boxes are.
There are a few issues with 'verts. One is you lose a lot of compression by having a soft top. The bass waves will use a lot of their energy to flex the top, therefore making it not as loud as what it would be if you had the exact same system in a coupe. The other is trunk room. Its a 3 series so the trunk is small to begin with. Then you have to have a place to store the top, which takes up room. There isnt much room left.
That is why I suggest a custom box. A good shop will take into account the sound characteristics of your car and make the box accordingly.
Good luck and welcome to the forums btw...
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