Blackburn have developed their Outpost Elite Universal Seat Pack (USP) to work with dropper or standard seat posts. You'll be able to carry a bevy of cargo, and it'll be stable enough to not swing around as it carries your gear. The USP is a two-part system:
Part 1 is a T6061 Alloy Wing that affixes to your saddle and works with a standard or dropper post.
Part 2 is a removable and waterproof dry bag. The bag has a scuba-style air bleed valve to assist with compression while off the bike. The wing stays on the bike, the bag comes off and allows you to pack, compress and ride in the snap of a few buckles.
Weight - 626g
Volume - 5.25 - 6.95L