Chances are the dust is from the road. With an open top rear fender and gaps between the bags and the fenders you can't totally stop it. You can decrease it by pulling your seat off and stuffing foam (or something similar) in all the gaps between the rear frame and the fender. Make sure to leave a space for the tab in the back of your seat to fit back into the hole in the back of the rear frame.
Retired USAF, IYAAYAS!
2008 V-Star 1100 Custom Bobber project
2011 Crimson XR with Forged Highway Bars / Hard Bags / Passenger Backrest, Ness parts including Smooth Layback Plate Frame/Mount / Rad 3 Mirror / deep cut grips / drivers floorboards /shifter peg / brake peg / passenger pegs. Witchdoctor black rack, Lloydz filter / timing wheel / VFC III, 7Jurock windshield, Drag Specialties seat.