Sounds like all you need is a stuff sack, two straps, and two short pieces of pvc pipe to offset the bag away from the bars. The straps go around the bag and the bars, but are threaded through the pvc in between. I have found that setup to work well. If you want waterproof, a roll top waterproof stuff sack would be suitable. Maybe one of the Sea to Summit Evac ones. The whole deal can be dirt cheap.