Hi from Germany! I have done quite a lot of string quilts and it never even occurred to me to use a paper foundation. If I needed a long piece of string "fabric" I did that by piecing a larger that needed piece of string-fabric and then cutting out what I needed. If I needed really long strips I did that in increments that I then sewed together. That way I could use very irregular strips too - I just didn't handle the pieces too often so any bias wouldn't stretch. Never had a problem that way!