To make the shoulder armor I cut out a bit of a lathe using Mesh/Slice By/Curve. Then added an edge using Extras/Add Trim Lines. This gives a rounded border.
To convert this to a square border I drew a rectangle loft, then turned it into a Brush Stroke and pasted the armor curve into the Stroke. Turn on "Stretch" for the Brush Stroke and rotate into place with "Initial Twist"