We developed our Outpost Elite Universal Seat Pack (USP) to work with dropper or standard seatposts, be able to carry a bevy of cargo, and stable enough to not swing around while it does. 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 go ride in the snap of a few buckles.
Compatible with fixed or dropper seat posts
Wing mount keeps your load from swaying during pedaling
Removable dry bag features a purge valve to allow for full compression when packing
An external shock cord will allow you to carry wet gear, or flip flops outside the bag