There should be a track behind the back seat where the front edge of the boot slides in. If it were a boat I would call it the "bolt rope", it is a small piece of cord sewed into the front edge that fits in the channel and holds the entire front down. If the track is missing you'll have to start the junkyard/swap meet game to find it. It is similar to other extrusions I've seen for boat cushions.
The boot doesn't come out of the track too easily so I usually leave it attached and fold the boot to fit in the space left by the top (if i put it up.) I usually roll without because of the pain of swapping it in.