I have worked with all of the materials you've mentioned. The question you have to answer is "Are you willing to change the subs (number/type/specs) or the enclosure (type/dimensions)?"
If you want to stick with your current configuration, and you are satisfied with the overall sound and performance of the box, then you're pretty well stuck with it. And that's a good thing. But if you NEED more trunk room (i.e. to store a spare tire or other stuff) you will have to change the dimensions of the box, type of enclosure (isobaric, aperiodic, sealed etc...) or eliminate one of the drivers and it's enclosure.
As for building the box itself, all of the previously mentioned tips are right on. As a work around for your current box, You can also try 'shortening' one of the walls of the box on a table saw, and fabricating a new side, and adding some acoustic foam, or even fiberglass. This will allow the drivers to perform in a smaller box without sacrificing perfomance.
If you have pics from your existing install that would help us understand your challenges and give better advice.
Best of Luck