How to measure the air space in fiberglass box. Fill it up with packaging peanuts, lightly dont cram them in there. once full find/make a small cardboard or MDF box with one cubic foot internal. fill the one cubic foot box up with the peanuts from the fiberglass one, and keep count. that will tell you what your airspace is. from the looks of what you are doing, if you go to the middle of the rear seats high, you will have around 3.5-4.5 cubic feet. I am guessing.
Chop mat tips. if you cant get it to stick you are using way too much resin. I take a wide 2 inch black throw out brush, plastic bristles not foam... and brush on some resin to the surface I want to reinforce. not so much that it drips all over, just enough to wet the surface. then take a wide plastic spreader, lay or shoot if you have the gun

just enough that it stick and partially soak through the strands, then take the spreader and apply just enough resin to keep it slightly moist. If you do it correctly you can build it up nicely in one flash. (one flash means buy the time the resin cures). If you are still having problems with the mat falling off, take a piece of fiberglass cloth slightly bigger than the area you are chop matting, and lay that over your chop mat. Then you can take the spreader and squeegy out the excess resin and it will stick for sure.
you could do that whole thing, tub which you already made and fleece the top in one gal resin, maybe one and a half. resin is not what makes it strong, too much and it is just extra weight, and accually takes away strength.