An air filtration system for a mobile unit having a vent fan and a kit for forming the air filtration system. The air filtration system includes a rigid outer cover made, a filter box, and a vent fan installed on a roof of the mobile unit. An air inlet passageway is formed in the rigid outer cover so that the vent fan may draw outside air through the filter box and into an interior space of the mobile unit. The air inlet passageway may include a louver for allowing air entry while protecting the filter box from rainwater. A base plate has a central opening that is positionable around a vent fan housing and securable to a vent fan flange to form a flat surface upon which to receive the filter box and the rigid outer cover.