A rear-mounted aerodynamic structure for securing to a cargo body includes a three- or four-sided structure having a deployed position for assisting in improving aerodynamic efficiency of the cargo body. The cargo body can be the body of a trailer of a tractor-trailer vehicle or trailer towed behind a vehicle such as a truck. It can comprise a trailer body or other cargo body well-known in the art. A spoiler “flow keeper” mounted to a top surface of the rear of the cargo body, as additional aerodynamic structures or as a standalone spoiler for cargo bodies. The three- or four-sided rear-mounted aerodynamic structure can comprise any of a variety of illustrative embodiments shown and described herein.