I've got 2, a small one and I bought this Saddleman 1800 one for my cross Country trip in July. It holds my cooler with ease and the bag itself has an excellent mounting strap/system that runs under your rear seat. Overall this bag fits nicely between your top case and you, also most importantly has a back rest and pad included. Nice to give a little back support! Super reviews as well, I got the base model as there is some variations that have Harley dressings and looks.
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I had one of these, actually the larger size on my Roadstar Warrior. Very handy, easy on/off and held all my travel gear that needed to go into motel room at night.
My only negative experience was when I was in really heavy rain, even with the integrated rain cover, the stuff in the bottom of the bag got wet. I think it was seepage around the stitching but I could never figure out where. Solution was a heavy black trash bag inside.