We hold it onto the rack with surfboard straps that lock from Kanulock.
https://www.surfsurfsurf.co.uk/roof-rack...WBoCh38QAvD_BwEThe bag came from here
https://www.boot-bag.com/morgan.htmWe went through several wet days including a thunderstorm in Naples that had standing water up to the running boards & all stayed dry in the bag.
I also fitted a stainless eye bolt to the rear passenger floor & have a stainless steel rope with soft eyes on it so I can padlock the Hills alive cases to the car when on the back shelf.
