Solid foam is expensive so see if you can use a Joann coupon. I would get the foam to fit the chair and then measure for the cover. If it is the square foam with flat sides you can do it a couple of ways. You can measure and cut two squares the same and attach a long piece of fabric all around the sides. OR You can over cut to larger squares and just sew off the corners to make it square like a box bottom bag. I would put either Velcro of a zipper on the back of the pillow for cleaning.