that enclosure from everything car audio isn't going to be simple or lightweight..... the front fascia was 2 layers of 5/8 MDF....
anyways.... if you'd like i can make you a full fiberglass box... most i have mailed out are around 9-10lbs.... significantly lighter than any MDF box.... search my name or audioaddiction for pics and such or pm me and i can price one out for you.
dude, go with the shallow mount jl audio driver subs, they are extremly light and will beat!!! i had 2 12's in my silverado with a slim box, and it was dazzling when people thought i had 2 15's back there.
Saving weight in your audio systems has it's trade offs. Lightweight, fiberglass enclosures resonate a lot so if you're going for sound quality, I would recommend against that option. You can do a custom box out of carbon fiber but that will be way expensive. What are you trying to achieve with your sound system? What is your current sub set up? Sometimes you can go with less to get more. For example, I have a single 12" Pioneer Premier sub in custom MDF ported enclosure that weights just 41 lbs with sub. The box is built to spec and I have hit 141 DB SPL and I have the entire system tuned for SQ. Answer my above questions and I'll provide the best options for you.
right now i have a kicker l7 12" in the kicer box it came in and im not really a speed freak or racing i just want a nice car and a good sound system the fi btl or a solo x is something ive always wanted but the cubic ft are 5-8 and i dont want to take out my back seats but if i have to i will