If you can sew make them out of old denim pants legs. stitch a zipper across one end and stitch the other end closed. If your ambitious include a liner of a dust proof material so you can use kitty litter and other dirty fills.
I have one of these and like it a lot. The folding feature gives it extra height for its size and works nicely draped over the edge of partially rolled-down car window. The strap is handy and lets the bag work as ballast for your tripod too.