Apidura, makers of dang tough bikepacking bags have evolved their  Backcountry Saddle Pack to be compatible with dropper posts. This is thanks to a nylon sleeve that glides up and down the dropper stanchion while securing the pack to the saddle rails. This pack has reinforcement in the form of a skid plate in case it hits the tire at some point, a loop for a light on the back, reinforced stitching, and weighs a mere 265g with a retail price of $156. See more at Apidura.