An attachment device for securing a flexible sheet within a channel member via an insert member of generally inverted V-shape wherein the latter is made of a ductile but rigid material having a central longitudinal portion of reduced rigidity that serves to define two adjacent wing portions and permits the subsequent plastic deformation of the insert member into its installed shape after its insertion, together with the flexible sheet, into the channel member. A method for securing the flexible sheet within the channel member, via the ductile but rigid member, is also presented.