How to cut out part of camera's view?


I am trying to make a portal system. Everything is working fine but there is only 1 thing remaining. I need to cut out part of the camera’s view. Here is a better explanation:

That blue part is what I want the camera to see, but I am not sure how to do that. I want to cut out everything else from the view and only see the part that is outlines in blue. How would I achieve that, I tried searching but no luck. Any help is appreciated. Thank you.

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