RaClips (from Waxwing or GrandBois) are perfect for this, as long as your rack has 5/16" tubing (Nitto, VO, and Soma rando racks all will work) and room on the side rails. Note that the bulges where the rack's cross-piece is welded to the sides (and, obviously, the cross piece itself) limit where you can locate the RaClips on the bag, particularly if you want to be able to move the bag from rack to rack. (In one application I have filed the plastic of the clips to accommodate the weld for the cross-piece.) Having an appropriate sized punch (a bit over 5mm) is extremely helpful for installation: you need to be able to make a clean round cut in the canvas of the bag, as well as the stiffener. The Waxwing
instruction booklet documents the process well. I've never had a RaClip come loose, on either side; clips + the leather slip-piece on the tombstone are shockingly solid even off road. Very fast on/off too. A nice finishing touch: a piece of high density foam (5-10mm thick?) cut to size of the bag's base, which gives a flat and quiet 'floor' to the bag once compressed into the top part of the clips.