Window seat cushions are best made out of a heavy, sun-resistant material, such as twill or a heavy cotton/burlap mixture. While these materials tend to be rough, by the same token, they are also long-lasting and UVA- and UVB-resistant. Both of ultraviolet rays can weaken material and cause it to age prematurely. A light material, though more comfortable will age remarkably quickly after extended sun exposure. If you are set on using something light, you might want to try a man-made material, such as breathable nylon or rayon.