Window seat cushions should be cut from foam pads that are about 1 1/2 inches thick. You can find this type of material at a local upholstery store or at a local automotive interior specialty shop. They tend to have various widths and sizes of cushion foam. Don’t forget to take good measurements of the seat area, as the cushions will not be glued onto the seat. Therefore, you need them to fit properly so that they hold themselves in place.