This tiny, but necessary, accessory does what it's supposed to do: block light from coming into the eye-piece and throwing off metering. Using your finger to block the light would work just as well, but is sort of limiting. So we do need something that will stay on by itself and, unfortunately, this is it. This little piece of rigid plastic fits most Nikon cameras with a square viewfinder, including my D-80.
Having said that, it is a horrible design. The original came threaded into the original Nikon strap and had to be removed in order to use it. This, of course, required detaching the strap from the camera and working your way through all those plastic loops--no quick task. Once off the strap, you're not tempted to re-attach it, which explains why you lose it between uses. There are so many things that could be changed about it:
--it should be possible to use it while attached to the strap;
--it should be possible to use it without removing the rubber eyepiece;
--it could be less clumsy to slide in;
--or it could be made of a more flexible dark rubber (like some Canon ones which you just have to stretch on);
--it could have some sort of loop on it which could enable you to attach it to a key chain or some other storage;
--it could have a bright colored rim or something which would make it easier to spot/harder to lose;
--it could be less expensive or come in a pack with several in it;
--or it could be larger so it's harder to lose.
There are any number of ways that this design could be improved, but since this is all there is, this is what we have to use.