I have good results with used dryer sheets as my foundations. They are lighter than muslin so make for less bulk.
As the used dryer sheets need to be pressed before stitching, they can sometimes stretch, so I plan to trim to a set size after the strips are attached. The brand I'm currently using trims to about 9" x 5". I've also had good luck piecing over-size diamond shapes and then trimming to size.
To answer an earlier question: Yes, it is just strips of fabric sewn to a foundation of some type. (I've also used paper, but then you must get ride of the paper.) For those of us who are compulsive scrap savers, this is a good way to use scraps.