Abstract: In a system for covering an opening in a structure, the opening having a periphery, the structure having sidewalls surrounding the opening and extending along the periphery of the opening, and the system including a frame system mountable on the sidewall so as to surround the opening, a flexible sheet for covering the opening, and connector parts for securing the sheet to the frame, the improvement wherein said connector parts comprise a plurality of button members retained by said frame so as to be movable in the direction of the periphery of the opening and to be inseparable from said frame, each button member having a button protruding from said frame; and a plurality of receptacle members each secured to the sheet and having a receptacle in the form of a pocket for securely but detachably retaining said button of a respective button member.