Singh-Ray has 4"x6" GNDs, but they are very expensive. Don't know if there is a holder that fits them.
Some use these hand-held against the front of the lens which avoids the problem with smaller GNDs of getting fingers in-frame.http://www.singh-ray.com/grndgrads.html
This filter is a bit out of my price range I'm afraid. I have been told that the Cokin Z pro holder with the Hi Tech 100mm filter will not vignette so I will probably go with that one.
Thanks for your help on this