I bought a cheap, solid black comforter at K-Mart or something along those lines...
If you don't have anybody in the second row of seats, you just tuck the edge of the comforter over the seats and lower the head restraints to hold it in place. It keeps pet hair and grime off the carpet - which seldom gives up pet hair without an arduous battle with the vacuum cleaner, lol...
A full size comforter will more than adequately protect the cargo area of the 5. I think mine was probably 15$ or so..