OK, now do OSB or 3/4" MDF/cheap ply with a single 1" brace in the center.
It's your money, and ultimately you need to weigh your priorities here, but as far as I'm concerned you're not making furniture or something that you'll need to look/access inside so apart from nicer veneer if you plan to stain/poly I don't see a lot of incentive to quadruple my cost of lumber on a car subwoofer enclosure.
Ahh, I gotcha. I'm going with birch for some weight savings and I'm going to put some poly on it to make it look nice and waterproof it. I also do want to be able to take the top off so I can access the port Cutout from the front down the line if/when I decide to run it ported.
Let's not talk about OSB, that shit is awful. As far as MDF, I could go 1" MDF as well, but I'll be adding an extra 80ish pounds to the car over the Birch. And a regular plywood isn't bad either, but I'm still paying 120/sheet for decent void-free 9-layer 3/4. And I end up using an extra sheet, so the cost is pretty much the same.
I'm looking at adding a simple cross brace inside the enclosure, but I haven't fully committed to it because of the odd shape. There will be at least 2-1" dowels inside the enclosure on the top, splitting it into 3rds a bit behind the sub cutouts. There may be a front to back dowel as well, but if I decide to follow through with the port, I have no real space for a dowel in the center. Keep in mind that the bottom, back, and sides will all be bolted to the body or basically locked into place once the doors are closed. The bottom is on the rear seat pan, the back is the trunk/rear crossmember/strut tower area, and the sides match up to the doors once closed(expanding foam will fill the gaps in the future, not the cheap "gap fill" stuff either. I have 2-part hard cure foam available from work.