I would be worrying about too much weight for the Givi (I assume) rack. Seen reports of broken racks. I had the black Givi luggage rack installed on top of my V46 topcase.
On long trips I put a Cortech cylindrical tail bag inside the V46 as a liner. It's a near perfect fit. When I've bought too much crap along the way I take the liner out and mount it on top of the rack, packed with lighter stuff, and I have the empty V46 to put other stuff in. Linky. The price should read $89.99