I would use a very inexpensive, lightweight muslin. Wash and dry it first to shrink it. It shouldn’t add enough weight to make a difference and it will last. Featherweight iron on interfacing is not woven and so will eventually disintegrate, losing its ability to prevent bearding. The muslin I am thinking about is very light in weight — not the same kind of muslin you would use as background fabric in a quilt (e.g., Kona muslin).