OK. I've got some more info for you to go with. The depth of the driver is 5.1 inches (The Stryke driver mentioned earlier). When you get a chance, measure the depth of the spare tire well and let me know how far 6.75 inches would stick up above the trunk floor. I would do this myself but my car was totalled about a month ago.
If you are going to mount it in the spare tire well, with a the minimum depth allowed by the driver, you will end up with a 1.35 cubic foot enclosure. I forgot to mention that I would recommend using sonotube for the enclosure. If you are not familiar with Sonotube, it is a cardboard material that cement workers use to poor large columns. There is no issue with the strength of the sonotube, it will have less resonances than all but the heaviest, most well braced boxes. Basically, you will cap both end of the tube with some 3/4 MDF. In between these two caps (top and bottom) I would use some 1" dowel rod that will attach the two caps and screw them together. This will eliminate any flexing in the MDF. The 24" version of the sonotube is what you will want.
This enclosure will probably stick out from the trunk floor, which can actually be a good thing, because it will allow you to build a false floor a couple inches above the current floor level. This makes some things very easy, first, you can flush mount your amp/amps in the floor. Second, you can run all your wiring under the floor very easily. The biggest advantage is that you don't notive ANY floor space being lost and it can be made to look completely stock.
E-mail me if you have any questions about how to build the false floor. It is much easier than my description probably makes it sound. davidnewman@excite.com
