Please Check my thread entitled Ultimate focus hood on this same forum. The material I used would be perfect for a bag bellows as it is light proof and inexpensive and can be found at Jo Ann fabrics for $5.00 a yard and can be glued using black automotive weatherstrip adhesive. The material is a vinyl-neoprene and the scrim material is a silver-gray ballistic nylon. The material is heavy enough to hold its shape, yet supple enough to collapse flat. The pictures I included show just how stiff yet supple the material is. Good luck and keep us posted on your progress.
Denise Libby