You can use zip ties to attach almost any basket to any rack. If you have a bag you like, I'd pick a basket to fit the bag. This is a Wald 137 basket and Timbuk2 medium messenger bag, and it fits nice and tight:
I prefer the basket up front, but you can do the same setup on the rear. When I have an extra large load (e.g. 2 bags of groceries), I can strap the messenger bag to my back and fill the basket with other stuff.
One other tip: for best handling, you want the weight of the rear load as far forward as possible without hitting your legs when you pedal.