I was looking for a way to secure a small garbage sack and hadn't found a good way, so thanks for pointing that out. I think it will work OK for that, tho I may need to alter the little vynil bag so it hangs a bit higher and is not in the way of the passenger.
I also found a small nylon storage sack about 11" long by 4" wide by 4" deep for using in your auto. It has an open top with several pockets and a zipper pocket along the long side. It also has some velcro hooks on the bottom and sits quite well on the storage shelf behind one of the seats. Got it from Target.