I just got the King Kargo Cage for my mtn bike due to the dropper. One ride with it so far and it was great. No noise or other issues. The bag is not super easy to insert into the holder but once it is there it stays put. I think everything I have in it would be accessible without removing it from the cage.
I left the King bag on the upside down bike for the drive to the trail head and there was no problem.
Plus you get one of the best bottle cages.