I built up the thickness of the back half of box using 3 types of cloth. I started out with a 2oz. woven cloth, don't ask me what the 2oz. means because i dont know. I ended up building thickness with a 6 oz. matting cloth. this built thickness very quickly. I ended up buying a a heavier woven cloth I think it was 8 oz. or 10 oz. and used some of it too. here is what i see as the difference between them. The woven fabrics conform much more easily to irregular shapes, but the heavier one has a harder time laying down flat when folding it over a sharp corner. forming it to corners when laying it on the inside of the box worked well with the heavier cloth, but wrapping it around the outside of a sharp corner was hard to do. The lighter woven fabric conformed easily to most shapes but built thickness and strength very slowly and took many layers to equal the thickness of one layer of the heavy woven cloth. The chopped matt built thickness faster than any of them, but would not follow sharp bends in the shape of the box easily.
Once I was happy with the thickness, strength and fit of the box, it was time to locate the MDF rings that the speakers would screw to. I had to make sure the back of the speaker would clear the inside of the box and outside would clear my roll cage. also I had to calculate to determine the volume of the box and install a baffle so that the subwoofers would have the right volume of air space to work properly. In my case I needed about 1/2 a cubic foot for each of the 8" subs. To figure that out, I marked a cardboard box with a line and filled it with 1/2 cubic ft of foam packing pellets. poured them into the speaker box and put a baffle in to limit their volume accordingly.