Our latest effort to combat COVID-19 is a box fan pulling air through a HEPA furnace filter with a 13 MERV rating. You can find the CAD and more on GrabCAD at https://grabcad.com/library/fa-20-hbf-1. Total cost is under $100. We estimate it will turn over the air in a standard classroom about six times per hour.
We used 1/8″-thick neoprene, adhesive-backed foam tape to create the seal around the openings. It might be helpful to have different thicknesses available to allow for variances introduced when cutting the plywood. The filter is held in place by suction. The fan can be turned to different orientations to get the best seal.
Future upgrades include a handle mounted to the plywood mid-section for carrying, elastic cord to hold the filter and fan in place, and 3D printed inserts to create a better seal for the fan. We recommend purchasing your fan then taking measurements to make sure you get the best seal. Our unit was designed to use a Lasko COOL Colors Box Fan (Model B20301, SKU 0 46013 45492 8) available at WalMart.