By cutting some N95/best available filter fabric and fitting internally over the exhalation valve. When flying I’ve worn a Korean nanofiber mask over the top for added social protection from ignorant flight attendants and airport staff who otherwise might ask me to remove it because they can’t see the internal exhalation valve filter. Korean commercially available masks are great by the way and there’s no shortage over there.

In situations where a respirator isn’t viable or necessary a some masking tape over the nose bridge wire area is quite good for reducing or eliminating glasses fogging and providing a better seal.