Awhile back I answered a question on how to adapt a DR6 right angle finder to a D700. I purchased my DR6 adapter from Gadget Infinity a seller on eBay. It cost $12.95 It was called a Viewfinder Magnifier Adapter for Nikon D3, D2, D2X, D2H. It just screws into the D700 stock eyepiece and the flat end is where you mount the DR6 by sliding it on. The second washer is to secure the items together and rides between the D700 eyepiece and the flat end.
It seems Gadget Infinity, gadgetinfinity.com ,no longer stocks this item, but it looks very much like a Nikon DK-18 to me. So am am posting a picture of what I had purchased to help others find a suitable item for their DR6. The item is black plastic and has worked very well with my D700 and DR6 combo.

Bob Baldassano
My Nikonians Gallery

"Nikonians membership - My most important photographic investment, after the
camera"

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Attachment #1, (jpg file)