For you guys having difficulty with filters for large lenses, one thing I did was get lucky on Ebay a couple of years ago and found a complete set of Kodak Wratten series filters, they are actually 6 inches x 6 inches and there is about 24 filters in the pack of various styles and colors, but once I compose my shot, I just take the filter gel out and hold in front of the lens for the exposure, now if it is windy or rainy, it won't work as the gel is thin enough to be affected by the elements, but on those calm days it works just fine, you just have to make sure it is flat when your holding it front of the lens..
Works for me, could be a solution to the filter problem for some.
Dave