Is there a round eyecup for the Nikon D600?
You can use the Nikon DK-22 adapter to convert to a round eyepiece and eyecup on the D600. The adapter slides over the square viewfinder and give you the 19mm female thread necessary to mount a round eyepiece. You can use the 19mm Nikon FM2 eyepiece, which is a standard eyepiece for the Nikon FA camera. The eyepiece can be fitted with a round DK-3 eyecup. The rubber eyecup will cover a small part at the top of the LCD.