Question Post Processing on a part of frame

I am trying to make some post processing effect on a part of the frame.
I want to increase the contrast on the center of the frame. since Post Processing works and follows the camera I want to change the contrast on a specific circle that I would choose its diameter beforehand.
What I tried?
I tried working with two camera but no success.

I was wondering if I can apply Color Grading on a specific part of the frame no matter where my camera is looking at?



Check Attachment: I want to have effect on the part this inside the circle no matter where the camera offset goes to

Thanks