I use 100% cotton for these pocketed masks and I sew in a wire nose strip that can be molded again and again for a tight custom fit along the bridge of the nose and under your eyes. This creates a tight fit and helps in keeping your glasses from fogging up. The pocket is for your own filter that you can slide into place. It has an extra large opening for easy accessibility.
In my own research I have found that hepa filters are dangerous to put in a mask because of the fiberglass that they are made from. I have been using wet wipes inside my mask. Paper towels and blue shop towels are another option. You want to use a non woven material.
Before using your mask for the first time, please wash in warm water and dry. Always wash your mask after every use and possible exposure. Do not touch your mask while wearing it (only handle by the drawstrings).
*Please use your own judgement and research for yourself appropriate filters. Be safe, be well!