In a lot of tests Corsi Rosenthal boxes tend to outperform most commercial offerings and they are very cheap as they are a box fan duct taped to some MERV13 filters. Effectiveness is a lot about air exchanges an hour.
I have one of these set up on each floor of my two story house to cut down on particulates from dust, pets, and cooking. If they also help protect from disease that’s just icing in the cake.
