The goal of the sketch modeling is to permit a really quick modification.

 

First I start with the structure of the seat :

 

My option was to positionate a character on the seat. That's permit to have an idea of where are the really important zone of comfort

For the modeling, I've used the blend curves, that permit a really quick modification, without be obliged to rebult everything each time you need.

I've begun the creation of surface with a first volume

I have to anticipate the minimum of thickness of foam on each part of the seat.

I've rebuilt, and obtained this volume

And I've finally improved the quality of my surfaces in order to obtain this result

And the evaluation of curvature