A very late thanks for your comment, John.
Back to the initial question, I am curious how the DK-17M is getting lost on the bodies that have the round eyepiece, as the eyepiece is supposed to be locked in place by opening the viewfinder shutter, preventing rotation? There is mention of shallow threads - have the threads perhaps been stripped or is the fit not so good?
I also found a very useful old DPreview thread about how to correctly attach DK-19 eye cup to the DK-17M:
https://www.dpreview.com/forums/post/26558579There are also some useful information about a part number down in the thread.
With my adaptation to the square eyepiece of the consumer bodies, I have however just slid the eye cup over/behind the rim of DK-17M, keeping the original rubber on the DK-17M.