The problem with the commercial slings is they are too big and bulky. You have to wear a bum bag to carry it when you go paddling, or stash it in the bushes.
You can make a perfectly good sling from a bit of 2" webbing. The finished product is compact enough to put in the pocket of your boardies. Sew (or tie) a couple of small loops in each end. Pull the loops back over the webbing to make bigger loops, slip the big loops over your board and away you go. Maybe practice with a couple of tied loops until you get the length right.
The best tie downs if you are going to leave your board unattended on the roof of your car are the lockable ones.
http://www.stickprotection.com/The best non-lockable tie downs are made by Sea to Summit. You buy them from Rays Outdoors for about $25-30 a pair. They are rated at 25kg each and certified to Australian Standards. The metal buckles have neoprene covers so they don't chip your paint.
The tie downs will not damage the rails of your board, but you might if you over tighten them. Don't do that.
