Thanks. I did use a monopods with. RRS monopod head. I left the mount ring loose so I could twist the camera to stay level as I planned on the monopod.
I rented the 200-400 3 times. I used it hand held for aircraft in flight. For me it was too heavy for panning, and I had a hard time zooming in and out while panning. So I decided not to buy it.
But you get great results with it.