A cover assembly adapted to be fitted over the open top of a bin includes a pair of spaced apart, domed rigid hoods releasably attachable to upper portions of the hopper whereby an elongated opening is defined between the hoods of sufficient width as to provide access to the hopper. A flexible closure member spans the opening between the hoods while supported on trackways extending along marginal edge portions of the hoods which bound the elongated opening. A pair of rotatable rollers are attachable, one at each of the opposing end portions of the opening. One roller is spring-loaded, upon and from which roller the closure member is wound and unwound during travel of said closure member over the elongated opening. The other roller is connected by pull cables to the closure member, and powered to provide for travel of the closure member.