Blackburn developed their Outpost Elite Universal Seat Pack to work with a dropper or standard seatposts, carry a bevy of cargo, and be stable enough to not swing around while it does.
Features
Designed to work with or without dropper posts
Harness that securely carries an inner bag
Waterproof dry bag features a scuba style air bleed valve so that you can pack your bag as tight as you need
Welded seam construction for reduction in bulk, weight, materials, saturation, and leakage
Capable of carrying a bevy of cargo, and stable enough to not swing around while it does