How can I form a rectangle / best fit shape about this plot using a matlab?

I tried using spline function but it is not able to form a closed shape. I have to do this in a continuous loop so an interactive option would not be feasible.

I need a line as highlighted. The points are very dense.

Any help would be highly appreciated.

[Jan]

What exactly do you mean by "rectangle / best fit shape"? Would a rectangular bounding box match your needs? Do you want a convex hull? Should it be a rectangle considering the rotation? Please do not less the readers guess the details.