I made a translucent lens cover for the front lens of the scope. I took a transparent lens cover and roughened its inside face with steel wool. Then I painted the roughened surface with clear nail polish, stippling the finish by daubing up and down with the tip of the brush. In other words, I made an optical diffuser. As it lets in no coherent light to form an image, all you see is the crosshairs projected out to whatever your scope's no-parallax distance happens to be. (On the scope I favor it's a fixed 150 yards. On some it's adjustable.)
And this is what you see when you look into the eye end. You see the crosshairs floating in space and no distracting image. The way an OEG works is you look at the target with one eye and the crosshairs with the other and your brain merges the images from the two eyes into one. Thus, you see the crosshairs superimposed on the target. You need binocular steroscopic vision for this to work. (In other words, your eyes work together well enough to produce a 3-D image when you look at things normally. That's nearly everybody.)
There is a pitfall to watch out for when using this or any OEG. You must fire just as soon as you see the crosshairs on the target. Look at the sight picture for much longer than that takes, and the effect called "phoria" makes the reticle wander with respect to the target. Those who have tried various OEG's and pronounced them worthless have tried to look at the sight picture at length and judge it carefully, target shooter style. As a result the aiming reference slews away from the target and they miss.
To repeat: If you pause to concentrate on the reticle, the reticle slides away from the target. You can delay the onset of this slip-sliding visual effect by deliberately concentrating on the target, NOT the reticle, but that doesn't get you a whole lot of extra time--a few seconds at most. Never focus your attention on the crosshairs. Over all, I have found the best way to manage an OEG is to look at the target with both eyes, then raise the sight into your line of vision just a moment or two before you fire. Otherwise, phoria effects will put you way off target. Be warned! Get the shot off QUICK and SOON!
You may notice divergence between where you hit with the cover up and the cover down. Worse, the difference may vary from day to day. This is one of those things you will have to try out for yourself--everyone's eyes are different. The OEG concept is good for fast coarse aim--and nothing else.
Okay, nobody said the OEG aiming concept is perfect. It was something that was used years ago in the early red dot sights. I have in my collection one of the old original Singlepoint dot sights, which works in just the opposite way from the goblin cap. You look into a blacked out tube and see a glowing red dot suspended in space, which your other eye superimposes on the target. Instead of a black aiming reference in a bright field, it gives you a bright aiming reference in a black field. It behaves just as scandalously with respect to phoria effects.
The neat aspect of the goblin cap arrangement is if an OEG isn't appropriate for the shooting you are doing, then a scope probably is, and you get your scope back instantly just by flipping the lens cover out of the way.
Why did I name it the goblin cap? I have it on there in case of goblins. They'll get you, if you don't watch out.