idleprocess
Flashaholic
There'a very little evidence of the virus - alone - being airborne. It certainly is in mucus aerosols, but those are way easier to trap than the virus itself. Larger particles that'll fairly easily get stopped or stuck by a filter. Even a 4 ply home made mask from T-shirt textiles will provide some protection against casual exposure if you jog behind a spreader that sneezes.
If you want to DIY a mask, it's still fairly easy to step up from tshirts or coffee filters. HEPA filters intended for air purifiers or vacuums are good candidates here, even if you buy a filter for an air purifier or vacuum you don't have, just to get the filter. If those are not available, the fabric from the vacuum bag itself will do. I'd take those above a coffee filter, but if I had nothing better, a coffee filter between right woven textiles will do as well.
For any kind of DIY-solution, I don't think the choices in filter material is even the most significant factor, but rather that you get it tight enough to not just drag in unfiltered air around the mask itself. If you're high risk and worried, use masking tape around the mask, to get it tight to your skin. Masking tape can also be used to hold the straps of a DIY or normal mask, to avoid touching your face to adjust.
I've heard that two layers of blue shop towels slipped into an ordinary cotton homemade mask are nearly as effective as a N95 mask - and probably more than sufficient for casual use.
I've pondered cutting up some HEPA filter bags for a vacuum cleaner, but I gather those can have some nasty contents such as fiberglass and other chemicals you'd rather not be breathing.