I just purchased the Ortlieb medium sized dry sack from Wayne at TheTouringStore.com. Not only will it hold my sleeping bag and tent comfortably on top of the rack, it comes in "you can see it from space" yellow, which I'm counting on for visibility on the road. I also plan to use it as a jack-of-all-trades waterproof duffel bag for family camping, commuting, gym, etc.

Thanks to the posters re: garbage bag tip. I'll combine that with the 1-2 gallon Ziplock bags to compartmentalize the "big bucket" Ortlieb panniers.

Ken