The curves don’t touch each other? So you need to Extend Curve and then find intersection points. However, your curves are not all co-planar so they don’t all intersect. You can project them to a plane and then it will work:
boundary_curves_2021Feb1a.gh (6.5 KB)
Getting rectangles from this is another step… Several ways to do it but it’s a little more complicated and they won’t be where you have them now.