Hi Angel,
I am in the process of building a fiberglass box for under the rear seat of my Ram quad cab pick-up, this may be one way to go for you...the fiberglass route that is. Do you absolutely want or need 2 of the 12's? You could make an enclosure in your spare tire well for 1 12" or 10" sub. That way you don't loose any trunk space. If you aren't worried about taking up the space a custom formed box could be made to house the amps and subs with a really custom look. I would suggest shopping around the stereo shops in your area and look at some of the custom things they have done. See how creative they are and their quality of work. There is really no limit to what you can do with the fiberglass route. Remember though, custom is expensive. The materials are cheap, the labor and time it takes is what costs so much. I do install for a shop in my town and because of the costs involved we don't get into the custom area much because most do not appriciate the time and work involved with the custom end.
Hope this helps a little.