I've never seen one in 6" x 6". All I've seen are the 4.5"x 5.25". I don't know what the diameter of your Nikkor 14-24mm is but if it's bigger than the 4.5" x 5.25" piece of glass then that is one wide lens.
The outer diameter of the lens cover on contact with the sun shade is 98mm so almost 4". 4.5" may cover it if I can get the form factor so the filter is within a couple of mm from the limit of the sun shade. Even still I can't find those glass squares here, only ND 3.0 Gels which kinda defeat the objective.