How do you make a 3D Holographic Sight that can't be seen unless looking through the sight?

Complex but answerable question. I need others input on how to make a holographic sight with a reticle that can only be seen if you are looking through the sight. In other words, a REAL holographic sight. In more detail, a holographic sight is called holographic because it uses light reflected off of glass to create a hologram that can be used for aim, but what is the point if it is 2D? A 2D holographic sight is just a glorified version of only 1 ironsight. You can't aim with it. Holographic sights appear 3 dimensional, but can only be seen through the glass due to the way the glass projects the light.

Escape from Tarkov did it, and I am sure many others have as well.

THINGS I DO NOT WANT TO SEE AS ANSWERS:

Please, DO NOT tell me this isn't possible, because it is.

"Make the sight only visible when aiming" No, I am not going to make a sight appear out of nowhere. That is a ridiculous and lazy solution to the problem and I can't believe someone would actually suggest that. It would be seen outside of the sight.

"Picture in picture" I have already received this response, and while it was a good and well thought out idea, it wouldn't work. It would look way too sloppy.

However, I have thought of my own idea to do this, but it would be difficult to make correctly. If there is an easy way to move and rotate GUI objects based on the location and orientation of the item it is supposed to be following, that would be great. My idea is that I could always have the sight reticle in the center of the screen, but have it a child of a frame that always follows the position of the sight on the screen. What this would do is make the sight only visible if the frame was in position of the sight. The only issue with this is that is if the gun were to move or rotate as the character say moves around or looks around then the reticle would stay in place. If anyone knows a way to program it to follow the gun's orientation and position that might work.

I have put a lot of thought and time into getting this to work and at this point I won't settle for anything less than this working well. I refuse to make the gun stay in a completely fixed position in the player's hand while aiming.