For people with beards, most respirator masks simply don't work. There's only one popular low-cost, beard-friendly respirator on the market, the Duxerity Resp-O-Rator. Unfortunately, they use a proprietary filter size, requiring you to order filters directly from them. As far as I can tell, their filter is a HEPA N95, but there's not a lot of documentation. If you want N100 or an oily particle tolerant filter, you're going to have to buy a much larger and more expensive face mask.

After returning home after the Camp wildfire in Butte County, CA, I burned through my stock of Resp-O-Rator friendly filters pretty quick. This is an adapter that will let you use standard filters. The only thing you need is an O-ring from. PartsmasterPro makes one called #217, and it's sold at Home Depot in the sink repair section. It's 1-1/16'' OD x 7/8'' ID x 3/32'' and there are probably alternatives out there. It seems to fit standard respirator filters just perfectly, assuming you can get a bit of pressure on it. In the attached photos, you can see that there are tabs on the 3M filter I bought that make this O-ring just a bit dodgy, but I took a while dialing in the clearances, and I believe it deforms enough to seal properly under normal conditions.

I've also attached my Fusion 360 file, so feel free to tweak the clearances and re-upload. I especially think adding support for larger gaskets would be a good idea. I thought about printing one with flexible filament, but I thought I'd rather have this thing up and running with a minimum amount of fuss.