last tour was on SS with aerobars with my down sleeping bag tied under the aerobars with clothsline. a 1x1x14 inch piece of wood (okay a stick), that i found along the road, served as support when i found that the clothesline was displacing the bulk of the bag and digging in, and consequently loosening the bag from the aerobars. i found that mounting the bag on top of the aerobars was also an option.
once again proving that necessity is indeed the mother of, if not invention, improvisation.