The "Bag"

That's how I refer to it.  That round, beer keg lookin' . . solid construction Nelson-Rigg black zippered bag.  It's flown off a couple times, but it never really hurt it.  It's got a rain jacket you can pull over it. During storms, it gets power washed from the rear wheel, but my shorts stay dry.  It has zippers on the ends to expand 3-4" more if needed.  It works.