63) In Viewport Top, create a Rectangle (shape). Remeber to make a square and align it to the center of the scene.
64) Go to its Modify panel and raise the Corner Radius parameter.
65) Establish the height to the Rectangle in Perspective Viewport (Z axis).
66) If you wish, enable the Display as Box option in the legs objects on Properties panel (right click). This will optimize the scene and improving your workflow if your system is slow.
67) Now create another rectangle in Left Viewport.
68) Convert it to Editable Spline, go to Vertex subobject and use Fillet tool to give the rectangle this form:
69) Now back to Perspective Viewport and select the Seat Rectangle (the first one), after this access Create > Geometry > Compound Objects > Loft. With loft tool we are going to create the Seat Border. Click on the Get Shape button and select the second rectangle we've created. At this point remember to select Instance option (under Get Shape button), with this any modification that the Shape suffer, Loft will automatically update the final format of the object.
70) I don't know If you noticed, but this border is with it's corners inversed. If this happen, go to Loft Modify Panel and inside Shape subobject rotate the gizmo.