I use a fade tourney bag, and i also fastened a discraft weekender bag to the back of the back rest. I used the same kind of velcro bands that come with the cart to hold the strap to either side of the back rest.
Works great for holding all my various putters and gives me a 3rd drink holder. It also allows me to utilize the "putter pocket" thing in the fabric for other stuff, usually a bag of sunflower seeds, some clif bars, and whatever else.
I also have a small lunchbox cooler, has a strap that i adjust down to its smallest setting. If you put the strap around the backrest and let it rest around the back rest and on the seat behind the hinge, it hangs perfectly behind the cart, under my weekender, and partially rests on the back of the bottom bag area. Makes long casual or league rounds awesome because i can keep 6-9 drinks with ice with me.