A filter assembly including a rectangular inlet frame having a rectangular inlet opening to receive a primary filter and optional a prefilter. The filter assembly may include a rectangular outlet frame and a housing body that is collapsible to have a flattened shipping state and an erect state. The rectangular inlet frame may also be used as a housing by itself to support and house primary filters and prefilters. A gridwork including one or more grid dividers may be used in the rectangular inlet frame to provide 2 or more openings for receipt of filters.
