Here's an assembly I've used with a 600mm. It is very effective and I have added a Manfrotto long lens support at times to prevent any side-to-side movement. No lens foot on the lens in question, so this 3-point attachment is not possible, might be useful for other applications.
_IGP1179 by tsjanik47, on Flickr
A cropped example using a 1.4x with the 600mm at 1/200s:
_IGP1004 by tsjanik47, on Flickr
_IGP1179 by tsjanik47, on Flickr
A cropped example using a 1.4x with the 600mm at 1/200s:
_IGP1004 by tsjanik47, on Flickr