For my use, muslins are too thin.
Jo Anne's fabric offers a wide range of fabrics, muslin, drill (often refered to as pocket drill,utility cloth or just drill), canvas, denim, pillow and mattress ticking. 100% flax linen is also good in a tight weave.
You need to take your micrometer to the store to measure thickness. Hold the cloth to the light to inspect for the tightness of the weave. Too loose a weave will let light through and if there are gaps for light, there is gaps for powder gases to blow by.
No matter what material you buy, be sure to wash it before use to eliminate the sizing that is put there to improve the feel on the bolt. The actual measurement is after the material has been washed and dried.
Be sure to mark the thickness of the fabric in several places so you know what you are selecting. Also mark the inventory number so you can go back and buy more if you find a patching material that works well for you.