Mirror Cut, Plane Creation?

I am trying to mirror a cut you will see in the attached part.  It would be mirrored about a plane that would exist between the two lines you will see sketched on either side of the part.  I know making another hole would be easier, I'm doing this for education purposes.  How do I do this?

While I'm at it, how do I create a plane through those two sketched lines?

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