The rear seat doesnt really weigh much at all. Just take the beam from the bottom of the seat and the side and back supports for the seatbacks. Also remove the latch on the back of the seats and paint black the foamy part now exposed. The center pivot can hold both seatbacks just fine. Then lay them back against the body hump and hook them on the inside to the pivot. They will be 15 degrees more back but will look fine. I think the bench weighs maybe 3-4 pounds and the backs 2-3 pounds each. People have sat in these and they are real soft and the seat angle puts people to sleep.
Just my thoughs. If you must, just junkyard a carpet section from the trunk and extend this where the carpet used to be.