holy crap has this gotten over complicated...
first of all, sorry for the misunderstanding, by your first post i thought you were doing this in a truck. it doesnt change much other then the difficulty of the fiberglass.
second, IMO you should def fiberglass for what you're looking to do.
third, you're running one xxx 15?
as a rough estimate, how wide is the shortest width of your trunk? (i.e. bottom of wheel well to bot whl well). Also, shortest height, and max depth you're willing to sacrifice.... given these things we can give you a low estimate of how much net volume you could get.
fourth, if a low ball estimate is 6 cubes, you should get a good bit more then that once glassed into all those little spaces (giving you displacement headroom for port & driver.
Glass or all mdf: if you have to make it first to measure INTernal volume the MAKE THE BOX QUITE A BIT BIGGER THEN 6.0 CUBES. Then later on you can add dead fill to the box to bring the volume down to 6.0 net (meaning your total internal volume is 6.0 + driver displacement + port displacement).
YES it will work. Its not really that hard. With one xxx in that area (of your trunk) you should have enough room for a 6cube ported box. You just have to get the measurements right to make sure your enclosure is tuned where you want it. Its NO MORE complicated then that.
Worst case, build or glass the box too big, calc & recalc your port (cant get around that even with a regular cubic box), then fill the box itself to where your port calcs will work.
YES IT WILL WORK. just a lil more work involved. Should not be a big deal. We can help, its just that you have the hard part of measuring out that enclosure.
I dont know any other ways to say it, i'm sorry if its not clear enough.