Rowan

I've taken to use helmet straps. I've had a few old helmets lying around, and the straps are one long length with buckles on the ends. You do need to remove one (preferable the male clip-in end) to unthread the strap, but it's a simple matter of using needle and thread to turn one end over on itself a couple of time, and to create a loop for the female buckle-end. The straps are long enough for a single one to wrap over my tent back on top of the rear rack and keep it securely in place.

I've also used very thin bungee cord usually available from ships chandlers, plus stainless steel hooks on the ends. They have proved very effective and durable.