I like the bungee and 550 cord idea around the waist and it works well to keep the cover on but what I don't like about it is that they pull the skirt up too high and exposes too much leg. Wait a minute, that does not sound right. I don't mind mini-skirts and exposed legs, as a matter of fact, I like them. In other words, the cover will only protect the bowl with this mini-skirt method.
@Taz has a good idea with the clips which would hold the skirt down over the legs and wheels, as long as the clips are stong enough to hold.