My boat isn't a 640, but is a 170TF (smaller boat like yours). I had one of the rubber bungie cord type straps on mine until it broke. I put one of the holders on the top of the rod locker close to the side of the boat and the other holder on the front lip of the dry storage compartment (put it there so I wouldn't step on it). When the rubber cord broke, I just replaced it with a regular bungie cord and have had it on there for right at 4 years now (the original strap only lasted 2 years)