I have two I built and have a tip for the cushions. Find a Stickley Audi store and go there, and if you can take your upholsterer with you, all the better. The shapes of the Stickley cushions are just right for the chair, and the pleating and welting details are, too. They cannot look like foam blocks covered in fabric, but must be nicely pillowed, and the right amount of batting is required.

Mine were done per the reproduction plans available from American Furniture Design, and have seat cushions with integral webbing. I prefer that to the loose seat cushion you will have. Consider a seat cushion with a flat or even slightly concave bottom, and not reversible.