I recently bought an Ortlieb Rack Pack to go across the top of my rear pannier rack. When the pannier bags are attached the Rack Pack works great: it clips into the two pannier bags and everything holds together nicely.
However, if I'm only using one pannier bag or if I'm using the Rack Pack on its own, I struggle to get the Rack Pack secured to the rear pannier rack. I'm thinking of using a bungee cord, but should I try something else instead? Is there a different kind of strap that is specifically designed for fixing things to panniers?