Koffee, I second what Rich says. When I first mounted my Arkel Utility Basket Pannier, I rotated the retainer clamp the "easy" way, ie using the short side, as the picture Rich posted shows. This seemed OK, but the bags seemed to rattle it bit. I rotated the retaining clamp to use the longer side, which requires a bit of force, but it locks the bag in place as intended. I can't imagine it ever coming loose accidentally. I can assure you this, Arkel seems like the type of company that will make it right whatever it takes. I love the Arkel suspension system for security and ease of use.

Gordy, the nice thing about the Arkel system is that it seems a bit flexible about forward/back positioning even if the bungees seem to want the bag in a certain place. I don't think you should need to modify your rack. In my case the limiting factor is how the rack is constructed. If the bag will mount where you want it but seems to want to gradually move forward as you ride maybe a small stainless steel hose clamp on the rack will act as a stop without interfering with anything else. Probably cost a couple of bucks for two and go on in about 30 seconds each. I can get my huge UBP far enough back to clear my size 12s. Care in packing and use of the compression system can help, too. Avoid putting anything in the lower forward part of the bag that causes the bag to "pooch" out forward. Just some ideas.
Regards,
Raymond