I have a Topeak DynaPack on my bike and my wife has a Koki Halfback. They are trunks that don't have a separate rack. They don't hold a ton, but they do hold a decent amount of stuff. Koki also has a Fullback that I think is big enough to stow a bike helmet. Most good seatpost racks aren't QR, and the ones that are are kinda wimpy as racks go.