I, too, like banana bags, but they likely provide more carrying capacity than you're after. I also use waterproof roll top duffles (mine are from Ortlieb, in small & medium) lashed on with Rok Straps, or the Kriega R20 or R30, that come with their own fixing system. These are waterproof roll tops as well. A rear rack may be useful for these, but is not necessary.