How should I fix my Ortlieb Rack Pack (medium, 31 litres) to my rear 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.