on my bikes i try to get a similar rack for each so that i can still use my panniers with any bike. the racks i have all have some hook built into the rear support piece where it welds to the main support of the rack frame. although on some of my cheaper racks, they are shorter and so my panniers ride closer to the crank, and i do get heel strike on one of my bike setups.

i have seen a fellow mount an home made hook - it looked like a cut piece from a coat hanger wire, and he held it in place with a hose clamp, seemed to work for him.