Particle board is not typically strong enougn to house 2 12s. Ply IS, bit you get the right kind of ply, specifically, birchply. It is lighter than most, more dense (ggod), but also a LOT more expensive (bad). It is an option, but not necessarily the right one for every application. I used it for 4 sides of the box I built in my last Miata. Left, right, front, and back were all birchply. The bottom was 50/50 birch ply and fibergalss (to form to the floor of the trunk) and the top was all fiberglass, except for the the woofer mounting rings. Worked great, and I saved a "bit" of weight.

MDF is by FAR the most common thing to use. If you are taking out the rear seats, you have a lot of options...pleanty of room to work with. You could (probably) keep the back sets if you wanted to...depends on the necessary space for the woofers...

Try Traffic Jams for some install ideas. Just go by the shop.

John