The present invention is directed to a rigid cover for a bottom dump trailer tank having a storage region with an open top used to transport bulk material. The rigid cover includes cover side walls having a top wall perimeter and a bottom wall perimeter and is configured along the bottom wall perimeter to correspond in shape with the open top of the trailer tank. The rigid cover includes a cover top connected to the cover side walls along the top wall perimeter and includes one or more ports in the top. The rigid cover includes cover fasteners mounted along the bottom wall perimeter configured to secure the plurality of cover side walls to the trailer tank so the traditional storage region of the trailer tank is transformed into an expanded storage region that allows storage of more bulk material than could otherwise be stored in the trailer tank alone.