Silly me... I found the stock eyepiece flat and discovered that it uses the same gasket I was looking for, so I swapped them. My stock gasket is unused and pristine, and I guess they are cheaply replaceable, at least compared to the DK-17M.
The DK-17M is much deeper than the stock optical flat. I'm not sure if that is a creature comfort to aid in glare reduction or inherent to the eye point required by the magnifier. But that is moot since I got my replacement gasket.
For those that might care about such things, I have a very old DR-3 right angle finder I bought for my N8008 20 years ago now. It screws directly into the plastic DK-22 adapter that is the heart and soul of all the D600 style eyepiece conversions.
The DR-3 also screws into the DK-7 adapter, which screws into the D3/D4/D700 style cameras. I suspect it might have come with a DK-7 stock?. As such, that RA finder works with all the cameras. The modern equivalent appears to be the DR-5.
The "compatible" RA finder for the D600 is the DR-6, but I am not aware of a gizmo to attach it to the round D700/D3 style cameras. As such, in this case the "wrong" RA Finder may be the better long term choice because one size fits all, with the addition of an adapter or two.
I suspect the main difference to a D600 user is that the DR-6 has an integrated metal slip fitting and will not break as easily as a DR-5 + DK-22. However, my experience is that RA FInders are easily bumped hard since they stick way out.
In that case it would probably be better to break the plastic DK-22 than risk serious damage to the DR-6 mount or the camera. And I've broken a couple DK-22's and tend order 3 at a time. Unless you use it out in the field where you might lose it. I use mine in situations where I am unlikely to lose it (not walking nature trails, etc).