Why don't you remove the rear seats completely together with the storage and spare tire trays?
This would not be a good idea as the covers will not fit properly once reinstalled. I can tell you have never removed the back seat material on our cars. There is no such thing as "extraneous" material as far as foam goes...The way to get the rear seats completely flat is to remove the seat covers and shave the extraneous foam away. But it doesn't sound like a flat cargo area is your real concern.