I use polypropylene to make inner cushions. Nice and light and easy to sew. By the way, I do this on my simple transport machine or my quick stitcher. It is specially for lightweight fabrics.
I always provide these types of pillows with a zipper. Very nice if you want to refill them again. You create the compartments, so the filling stays better in place. They are therefore particularly suitable for back cushions.
Of course it takes some practice, but it is not really difficult. Just watch the video.