I think your looking I the wrong direction. If you plan on buying the 500mm and already have the 100-400 I have used both quite a bit the only solution I have found is a gitzo 55 series with a wimberly head
The other option is a manfrotto 495B ( I think that's the one). The problem is almost impossible to avoid without being cost or weight prohibitive the gitzo sell for over a grand and weighs around 8lbs the manfrotto is cheaper around 500 dollars but weighs around 15lbs




These are you two best options as there is relly no light weight and cheap way to work with these lenses




good luck though

Coastal kid88