What can be used at the bottom of pots to keep the soil/hydroten from falling out of the holes in 5 gallon?

I line my pots with fabric. I like the smart pots you can buy(fabric), but I can get another plant or two in my environment using hard plastic 3gl pots lined with fabric. No soil mess. Going to be an easy clean-up. I hope, first time doing it this way. Good luck!