When I was doing this, I would tie the two loose straps together and place the backpack on the rack with those straps on the underside, but with the tips hanging over the sides of the rack. Then I would just wrap two bungee cords around the rack/backpack to hold it down. Worked for a while. It wasn't so secure that I could hammer up a hill, but it got the job done for the year or so that I needed it too.