I had these Nelson-Rigg panniers leftover from a motorcycle. They work thrown over the seat on my Liberty 150. The back strap goes behind the seat and holds them in place. There is an internal flexible panel that helps them hold their shape.
I only use them when I know I am going to need to haul a lot of stuff (like groceries). 27L each, but they hold about half again as much when un-rolled.
They have padding on the backside for going up against bodywork. I haven't noticed any scratches so far, but they won't show up as much on white as other colors.