The CR Box comprises four air filters commonly used in heating, ventilation, and air conditioning (HVAC); a cardboard base; and a box fan to draw air through the filters and toward the ceiling.

At its core, an air purifier is just a fan that blows air through a filter. In fact, if you’re truly in a pinch, taping five HVAC filters to a box fan (to create what’s known as a “Corsi ...