I have a seat/tail bag that carries all of my daily ride stuff. It is transferred between multiple bikes so I don't have to repack everything each time I ride a different bike. My 2015 FJ-09 required removal of the passenger grab rails to allow the bag to be mounted securely. Only issue is that 2 large holes are left on top of the tail section where the rear of the grab rails are attached. Unsightly and not exactly weather resistant.
Solution: 1-1/2" diameter steel washer (painted matte black) installed over 1-1/2" x 1/8" thick soft rubber washer, secured with the stock grab rail bolt. See attached photos. (NOTE: The aluminum mounting boss for the grab rail sits a little proud of the plastic tail section cover surface, requiring a soft rubber washer at least 1/8" thick to conform to the boss and seal to the tail section cover. A hard plastic washer will not effectively seal the opening. Soft rubber washer with a 3/8" center hole effectively seals the bolt shaft.)
While it may not be the most visually pleasing solution (more "industrial design/steam punk" than sleek...) it seals the openings, is fairly unobtrusive, rattle-free, and allows for quick re-installation of the grab rails.