a system for covering an opening in a structure . the system is for use with a structure having sidewalls which surround the opening and extend along the periphery of the opening . the system includes a frame which is mountable on the sidewalls and a cover sheet for covering the opening . the frame of the system has a channel formed therein . the system further includes a single piece flexible fastener , which is secured to the cover sheet , wherein the fastener is received by the channel for securely but detachably retaining the fasteners to the frame .