countourin perpendicular (normal) to courve

hey guys

I need to do countouring for this chair, but it has to be perpendicular to curve in the middle.

Normal conoturing doesent work for me.

I've attached (on the link) rhino and grass file, I've tried to do something but I can't get to the solution.

the sketch image is what I want to get in the end.

https://www.dropbox.com/sh/dceu4xo8etidhh2/AAAR6ePp4ufy7XwONaTi1MEX...