Sure. Just make the pocket slightly smaller than the mask and stitch it on the back by hand leaving one side open to add your filter.

Just cut a slit in it.

I've been making masks, pleated and fitted. You could probably add something to a fitted one, but not to a pleated one, because you wouldn't be able to fully open it if you add a pocket to it. Cutting a slit in it might make it unwashable since it will start to fray.

My cousin used fusible as a filter in some she made. She said she couldn't breath with it in the mask. She switched to coffee filters.

bk

I think some hand stitching of the cut edges will solve the fraying.

Theresa

Thank you so much for all your suggestions.! I will try them