I just received a 4 inch square Black polymer solar filter in the mail today and cut it to fit in the filter holder of my 600mm lens. I took one test photo and was starting to do another when the viewfinder started to blind me with sun light.
I just learned a valuable lesson. I cannot use a Black polymer solar filter in the gelatin filter holder of my 600 mm lens. Because it was in the concentrated light path in the lens, the sun just burned a hole in it. I just ordered a Meade #750 190mm Glass White-Light Solar Filter to avoid any more problems.