Your UL dry bags need the protection offered by the Revelate Harness and Terrapin saddle bag shell. They prevent abrasion holes from bike brake levers and the grit & sand thrown up by bike tires on and off pavement if you attached the dry bags by themselves. The Terrapin fits up to a 14L dry bag with some of it protruding a bit.

Also remember that UV sunlight will degrade UL dry bags over time as they are not inside a backpack anymore but partially in the sun all day long.

Another option:http://www.bikeforums.net/touring/95...l#post16892241