Now don't laugh too hard, but I just took the eyecup off of my old Canon A-1 camera and put it on the D100. That was 2 hours ago and it's still in place. I havn't field tested this mod yet, but it feels like it will stay. Now this eyecup is VERY old and the Canon eyecup is just stretched onto the D100, but I'm going to leave it there until it falls off. Like someone said above, maybe I'll add a dab of silicone sealeant to make sure it stays until I want to remove it.
Since this one "fit", I'm sure others fit even better. So all you people out there that have modified another eyecup to fit the D100 please stand up and be counted!
Another possibility is to buy the Nikon DK-4 eyecup (yes it's available at B&H) and mount it onto a Nikon 0 Diopter eyepiece (Yes also available)