I have had a topeak beamrack for over 2 years now. Its great, certainly never moved at all. Up to now I had an old camera bag zip tied on it but now have the Topeak rx expandable rack bag which has a couple of small panniers than can be used or you can zip them into the pockets if not needed.
Topeak instructions say it shouldnt be used with a carbon seatpost.
This is the new bag. It slides into a track on the rack. You press down a button to release it then take it into the pub/cafe whatever with you.I have seen quite a few Audaxers using them.