First of all, you want to restore the integrity of the chair. Any missing springs need replaced. The springs should be re-tied. This needs to be done with hemp which will not stretch. providing a solid seat. 2nd, the springs need to be covered with a thick batting. They used to use horsehair for this purpose. The purpose of the batting is to keep the springs from breaking down the foam. 3rd, the foam seat will need to be a dense foam, usually 3-4" depending on your preference of seat height and the structure of the chair frame around it. If this sounds too involved for your skill ability, an uphosterer will gladly do this for you. Then, cover with your desired material. Plywood has no place here. Good Luck!
