Abstract: An adjustable window structure formed of angle members connected to define a perimeter support structure in which the length of at least two opposite sides is made variable in size by using telescopically nested angle members on those sides, to allow the sides of the adjustable structure to be pressed firmly into contact with a window frame. The nested angle members are held together by resilient clamping members formed as U-shaped channels to fit over the edges of the angle members to maintain a high frictional engagement with and between the angle members. A flexible sheet extends across the support structure, and the edges of the sheet are folded around one edge of the angle members and locked into place by the clamping members.