Sundance (and most manufacturers) do not manufacture their covers. They buy them from a spa cover manufacturer. Your Sundance dealer can purchase the Sundance cover directly from Sundance’s cover manufacturer.
I know what is important in a spa cover: the thickness of the cover and the density of the foam inside the vinyl (Doc, please reprimand me if I am wrong).
The factory Sundance cover I believe is a 4-inch to 2-inch taper using 2-pound density foam. It is a good cover, and I would recommend going to your local dealer to get a factory replacement. You are then guaranteed a good fit and correct skirt length, which is also important.