I've had the medium Motofizz bag for bike trips for almost a decade, and like it a lot: http://www.aerostich.com/medium-moto...-seat-bag.html
(although it was less expensive when I bought mine). This is rugged, waterproof, and has all sorts of ways to pack stuff in or on it. It may be too big for what you want, because it's slightly wider than the back seat; I have the Victory passenger grab rails, so that keeps it off the lids, or you can, as suggested earlier, add 3M coverings. The Motofizz has built-in looped straps. I also use the Motofizz has an airline carry-on, or in the car trunk on certain trips. It is very rugged, capable bag, with lots of room, detachable shaving-kit type of thing, etc. Good stuff.
There are a lot of other bags and companies, of course. It terms of experience, I also have a much smaller Euro-Sport Hauler from RoadGear ( http://roadgear.com/tail-bags-c-23_31.html
). I bought this on sale last year, to go on top of a trunk rack, if I ever put one on. They also have a much larger bag, too. The Euro-Sport is not as rugged as the Motofizz -- lighter material -- and it has built-in bungee cords and hooks.
I had a RevPack tank bag about a dozen years ago, and I can attest that they make high-quality stuff (or at least they did). They sell seat bags, too: http://www.revpack.com/motorcycle/seatbags.html
EDIT: BTW, RoadGear is having a sale, ends today (02/14): 25% off on orders > $50, using code FEB14C