My strategy as I have plastic boats is to just place the boats upside down on the roof racks side by side. If you have glass or kevlar boats you will need one of those U shaped things as seen in your link to prevent damage to the boat plastic boats are fairly resilient but may warp slightly in the Sun. I use a very long set of tie downs I obtained from amazon.com find the lines to the front and the back of the car do absolutely nothing when I tie the boats down correctly however, they are required for by law in some states. Put both boats face down side by side. I take a tie down and tie the boats directly to the racks. I have the strap go under the roof racks inside the towers and over both boats together pinning them to the car and together at the same time. I use two sets of straps one for the front set of talks and one for the back set. You need to know that the the handles and the lines on the deck of the boat (both the strech and non strech kind) are not meant to support the weight of your boat and should not be used to tie down your boat. That's a reason why I don't rely on tying my boats front and rear. Also be aware that any cords or bands or rope or straps or whatever you use will strech if wet and if you drive through rain or they get wet after the boats are wet than they will strech. It takes a few minuets for this to happen so be aware you will need to re tighten all your lines. I put flags on the ends of my boats as they stick out beyond my car and it's required by law. When law requires me to have a line front and back I use kitchen string as I never rely on that to hold my nboats in place. I personally hate some fasteners that you can get on straps. I think they fail to often and I tie a knot at the end for security. Others I trust more. See link. For the tails of the straps I tuck mine in side the doors of the car but they will get your car wet if it's raining if you do that. I have not had an issue strapping it all directly to the roof racks. Some people use pool noodles but I find they roll around and can be dangerous if you use commercial racks. They areawesome when secured to ply wood. I use tennis racket handgrip tape around mine on the bars for a no slip finish. Some bars have rubber inserts which are even better. Does this help? Link to my straps http://www.amazon.com/gp/aw/d/B003ZNUCUM this next one is a link to the kind I hate as I have seen them fail when the mechanism gets old or a small pin or whire gets lose. http://www.amazon.com/gp/aw/d/B00I8BOF/ref=mp_s_a_1_5?qid=1399498335&sr=8-5&pi=AC_SX110_SY165_QL70

I like a long strap while not having the other piece you see with the hooks for a pickup on my car. Like the kind in home depot. I've ordered this kind 3x now and love them.

Does any of this make sense?