In the past I've used mesh window screen but discontinued that use because roots were attaching to the screening and I'd have to tear, or cut them away when repotting. I've also used coffee filters which allow drainage and keep the soil from exiting the pot but they also break down and disintegrate over time. Nowadays, I buy the compressed blocks of coco coir chips on amazon and use it in the bottom of pots and I also mix it into potting soil for aeration and I've used it for mulch atop the soil on some of my outdoor containers.
