The Kirk foot doesn't look very comfortable to use as a handle. Basically, you can't go wrong with Really Right Stuff. I've got the LCF-53 for my 600 II.
Neuro,
I'm preparing to purchase the 500mm f/4 IS II. While looking at lens feet, I remembered your earlier
post regarding Canon's and Kirk's feet for the Mark II versions on the 500mm and 600mm being too far forward and wanted to know if you had seen or considered this
one by Jobu Designs, as it extends
behind the mounting screws on the collar?
It looks like it might be a little difficult to use it as a handle -- even with my small hands! Probably have to put your index finger on the other side of the mount point. I have the 5D III and I'm not planning on getting the 2x TC (I will probably replace my current 1.4x II TC), so that extra bit at the end might not be needed.
-- Dan
PS. Before clicking
Post, I finally realized that this foot is
only for the 300mm and 500mm II lenses. For the 600mm II, they have this
one, which does not have the same design, so I guess you would
not have considered it!

Oh, well...