I have Jarvis Walker-branded straps, got it in Hartenbos for about R 15 - 20 a set of two, and they work extremely well.
And while you are about it, get an extra set or two - they work very well for rod holders too.
And there is nothing that works better to secure your rods to a roofrack, I now use nothing else. Quick and simple, much stronger than the rubber band stuff and not to mention the 'suction cup' rubbish.