How should I fix my Ortlieb Rack Pack (medium, 31 litres) to my rear panniers? 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?
I'd go with a couple of bungee cords, maybe the adjustable-length ones.  That's probably going to be more versatile and adaptable than something more specialized. 

There are some flat adjustable bungees, like these:


http://www.amazon.com/ROK-Straps-Adjustable-Loop-Thru-BLACK/dp/B008ETMV8I

That particular type is attached on either side such that you can open it without having the entire strap coming off the bike. Both ends are a plain nylon strap, and the middle section is stretchy.  I've used these for motorcycle luggage and I liked them.