45 which are positioned along the four sides of the structure thus the term 34 clamping bar 34 will be used to also describe the clamping ring since the cross sectional configuration as seen in the various views will be the same whether the securing means is in a continuous ring form or adapted as four separate straight members the clamping bar comprises an extrusion molded metal bar having a generally 34 l 34 shaped cross sectional configuration the vertical wall 46 defining a truss member and the lower horizontal wall 48 thereof defining part of the clamping bar securing means which is generally indicated at 50 integrally formed along the free edge of lower wall 48 is a strut member 52 angularly arranged to engage the outwardly extended shoulder 53 formed on the base of jamb wall 24 shoulder 53 is formed longitudinally throughout the length of frame 22 including an adjacent slot 55 to receive the rounded free edge of strut 52 therein whereby slot 55 and strut 52 combine to establish a pivot means so as to allow bar 45 to pivot into a positive clamping mode as seen in fig2 more specifically the clamping bar securing means comprises the lower wall 48 and a second juxtaposed.