As the corner Segment isn't symmetrical and needs to be mirrored, I would suggest you to pick the "corresponding one" by the Selector Operator
in the Spline Mat ID
mode. In attached possible solution vertical lines have the ID 1
assigned and horizontal lines ID 2
Segments in attached style are using the Pivot
Alignment and the precise placement is handled by using an empty Segment with Left
& Right Padding
values corresponding to corner Segment measurements.
Hope you'll find it useful.