In reply to Smelly Fox:
I have an Alpkit Big Papa, which I have used with my dropper post on my Giant Anthem SX (with a bit of extra protection to the post cobbled together I forget how). Its excellent, and was barely noticeable when riding, even though it does sway a little. Alpkit have recently added an "Exo-Rail" to the range, which is designed to stop sway, and to work with a drop post.
Some pictures on a blog I wrote here, though most are of my mate. Mine's the blue/silver/yellow Giant, nto the red/black Scott: https://www.wildernessisastateofmind.co.uk/bikepacking-the-south-downs-way
https://www.alpkit.com/featured/bike-luggage-seat
Post edited at 19:09