The video below features three mask tutorials. In the first, industrial designer Eric Strebel’s wife shows you how to sew a pleated mask that contains a slot you can slide a filter into; then Strebel shows you how he modified his shop mask for easy on/off; finally, he runs you through making a BU Mask, which is a cardboard mask (designed by Evgeny Maslov, freely downloadble plans at the link) that can also take a replaceable filter.
https://www.core77.com/posts/98433/How-to-Make-the-Cardboard-BU-Mask-Modify-an-Elipse-Mask-for-Easy-OnOff-and-Sew-a-Fabric-Mask-with-Insertable-Filter