An assembly for reducing the conveyance of material on a top surface of a secondary structure movable into and out of an opening in a primary structure includes a mounting member including a first portion coupled to an upper interior surface of the opening of the primary structure and a second portion situated on an exterior of the primary structure adjacent the opening. The assembly further includes a pivotable member coupled to the second portion of the mounting member and being pivotable between an open position and a closed position corresponding to an extended position and a retracted position of the secondary structure with respect to the primary structure, and a flexible seal member coupled to the pivotable member and movable therewith such that when the secondary structure is extended the seal member engages the top surface of the secondary structure at a location offset from the opening.