Localized post process

Hello, I am trying to achieve the localized post process effect like the one in the link below. It does seem to be lacking in information on how to fully set it up. Or maybe that technique no longer works? I need a way to achieve the effect in there, where post process effects everything but a small localized area.

They mention lerping between the post process effect and the scene color, but I have no idea how to do that. When I try and do it in the post process material it gives me an error that a scene color node can only be used by a surface material.

Any help is greatly appreciated!