HELP!!! Automatic Best Fit Lines!!

Hi guys,

I am still a learner of Grasshopper and now I don't know how to realize the modeling below. 

I have a lot of scanned models look like the one shown in the screenshot. I want to use grasshopper to automatically generate geometries from these meshes. The geometries don't have to be precise and can be just a simple extrution. I was thinking using bounding box and then I found there are a lot of U-shape models and T-shape models which can not be automatically generated from a bounding box.

What I think might work is get a cut line from the mesh and extrude. The problem is how can I get simple rectangles or shapes a little bit more complicate (like a U-shape or L-shape) from those points automatically? 

Thank you so much guys! I appreicate a lot.

PS:

The moedel attached only have dots because the mesh is to large. I only have dots from one surface for now, which is the edge A and I copied A to be B,C,D just to show the idea. The rectangle on the right is the final result I imagined.