A filter unit includes a body portion having a proximal end and a distal end. First and second engagement surfaces traverse at least part of the body portion. The first and second engagement surfaces include a first segment that extends substantially parallel with the longitudinal extent of the body portion. A second segment extends at an angle relative to the longitudinal extent of the body portion. A third segment extends at a different angle than the second segment. A laterally extending key member is disposed on the body portion between the first engagement surface and the second engagement surface. An engagement protrusion extends from the proximal end. The engagement protrusion has a first portion that includes a first radius of curvature and a second portion opposing the first portion that includes a second radius of curvature that is larger than the first radius of curvature.