30 a and 30 d are folded over at their edges at bottom side 26 b and top side 26 d to define the respective sleeves 24 a and 24 d however the frame retaining sleeves 24 a and 24 d converge at or are connected to one sleeve portion which interconnects side panels 22 a and 22 d to form a singular frame retaining sleeve 40 which retains the frame members 28 a and 28 d sleeve 40 may be formed by providing a tubular fabric or by folding a piece of fabric and applying a stitching 42 to its edges to connect the sleeve 40 to the fabric pieces 30 a and 30 d stitching 42 acts as a hinge for the side panels 22 a and 22 d the connections for the three other pairs of adjacent edges may be identical an upper panel 32 comprised of fabric 30 e may also be connected to the upper edge 26 d of each side panel 22 a 22 b 22 c and 22 d likewise a lower panel 36 comprised of fabric 30 f may also be connected to the bottom edge 26 b of each side panel 22 a 22 b 22 c and 22 d the upper panel 32 and the lower panel 36 are preferably made of the same type of fabric as the side panels 22 a 22 b 22 c and 22 d each module 20 preferably comprises at least the four side panels 22 a 22 b 22 c and 22 d with the upper and lower panels 32 and 36 being optional openings 38 may be provided in some or all of the panels 22 a 22 b 22 c 22 d 32 and 36 these openings 38.