Another option that may work for you (I just found them & posted a thread about them) are Surly Junk Straps. I've always had problems with my backpack moving around on a regular rear rack. A pair oft he Junk Straps solved that. I tie them so the bag faces front to rear & the straps go side to side, then I pull them snug & all is well. This is really the first strapping system that I've been completely happy with for my backpack so far.

If you do get the klickfix, please report back on how you like it. I was eyeing one of them a few months back, but never did pull the trigger.