Maybe this will be useful to someone on here... here's pictures of a rack i built to carry a normal backpack (or any bag for that matter) as a pannier.
Most of the items were bought at Home Depot.
I didn't end up using the bungy cords pictured.
I got the reflective tie downs at REI.
It's not light. But very functional. One change to the design is that i would probably cut the bottom plywood angled back to the vertical plywood (ie a trapezoid instead of a rectangle). There is rubber on the inside of the hooks and on each end of the locking lever. Everything was put together with locktite and is pretty much rattle free.
The pictures don't show it too well, but there are three straps that i use to hold the bag. Two horizontal (reflective) and one that runs vertical.
Its mounted to a early 90's roadbike that I am currently using as a commuter. There is just enough heel clearance with the setback shown.