Just wondering what everyone uses to attach things to the rear rack and keep them there? Do they made a good bungee webbing or perhaps a just a couple of bungee straps would be good enough.

I have a wide Surly Nice Rack rear rack and panniers on both sides. I have a water proof backpack that i would like to strap to the center of the rack to store a few things and use when i am off the bike. I am wondering of the best way to attach it. Seems to me i saw a small bit of bungee style webbing with hooks on it but i cant seem to locate it now.