I bungy cord a cylinder Dowco about 14" long. It holds about as much as both saddle bags and has an expanding zipper on each side if I need more room. It has straps and self velcro to hold it on the rack, but I feel safe with cords. It's seen lots of rain and has stayed dry. About $39.
I use the Harley touring bag and/or day bag (p/n: 94734-09 for both). The touring bag holds just as much if not more than my Dad's hard leather tour pak. It has a rain cover that rolls up and will fit over the bag, but the bag keeps water out without it. I looked at other brands but they all came out to about the same price but Harley's were bigger bags and comes with the rain covers.