It depends. My slide covers do flap at times, but not usually enough to be a major issue. It would depend on if your device was easily installed on taller rigs, and what the price point was. If the winds are really bad I will usually retract the slides on the side that is the issue.
If I were to buy and carry another new device to eliminate flapping of the slide top, it would have to be pretty unique. A really unique device would be one that I would install once and never have to remove.