i know how to stow it, is there any way of tieing it down?
+1 It protects the roof and holds it securely in place. Without the cover, the small screws in the seatback can scratch/dig into the roof and really damage the finish. Although I'm ashamed to admit it, I skimped on not having a proper cover for 3 years. The roof needed to be repainted twice. Now, although the cover is expensive, I've had zero problems. http://www.ricambiamerica.com/product_info.php?products_id=254562 It comes with a little zippered bag to keep it in too -- so its easy to store behind the seats or in the trunk.
In my car those screws pierced the cover and then damaged the roof. There's only one solution: buy a softtop!