of the invention will be described in greater detail below as with vertical rails 12 14 corresponding horizontal rails 24 26 are also surrounded by polymerized sheathing 32 34 respectively additionally this polymerized sheathing has an interior diameter that is equal to or greater than the exterior diameter of horizontal rails 24 26 as stated above this slip fit of the polymerized sheathing 32 34 about horizontal rails 24 26 will be described in detail later slip on structural fittings 36 40 are provided to removably engage horizontal rails 24 26 to each other and to vertical rails 12 14 these slip on structural fittings are preferably hollaender structural fittings manufactured by the hollaender manufacturing company cincinnati ohio t shaped slip on structural fitting 36 may be provided to join horizontal rail 24 to horizontal rail 26 and include an extra opening for additional vertical rails if desired t shaped slip on structural fitting 36 removably engages horizontal rails 24 26 and polymerized sheathing 32 34 by set screws 38 l shaped hollaender slip on structural fittings 40 removably engage horizontal rails 24 26 to vertical rails 12 14 l shaped slip on structural fittings 40 removably secure the rails to.