I bought some firm foam to make 4 chair cushions. It was 2 1/2 inches wide and I had enough to make a meditation cushion (see below).
I bought shelf liner to make the underside so that the cushions wouldn't slip around on the chair. It worked well! To sew using shelf liner (I bought "Gorilla Grip"), use a piece of tissue paper between it and the foot of the sewing machine.
I love my meditation cushion. It makes a church pew feel comfortable!