Sweep a rectangle along a curve

I am trying to sweep a rectangle along a curve with different widths. but I can’t get a smooth surface and i want the same thickness.
and actually, I am trying to build a very difficult chair using grasshopper so that I can adjust everything and it is inside the rhino, so if someone can simply give me a direction on how to build it, I would be very thankful!!! this is really beyond my ability. thanks so much!
190405.3dm (1.1 MB)
0404.gh (15.1 KB)

0404_re.gh (12.8 KB)

thanks!~ this is exactly what i want