Set a plane to camera viewport

 private void SetPlane(Transform plane) {
        float halfFOV = ( theCamera.fieldOfView * 0.5f ) * Mathf.Deg2Rad;
        float aspect = theCamera.aspect;
        float height = (theCamera.farClipPlane + 1) * Mathf.Tan(halfFOV);
        float width = height * aspect;
        plane.position = theCamera.transform.forward * ( theCamera.farClipPlane - 20);
        plane.localScale = new Vector3(width * 0.1f * 2, 1, height * 0.1f * 2);
        plane.rotation = theCamera.transform.rotation * Quaternion.Euler(90,180,0);

    }