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To prevent the top shelf from sagging; The two top left to right tubes needs to have two wood dowel (broom stick) (441mm long) inserted the full length. The dowel length center will need a ring spacer to contact the pvc inner top center if the shelf begins to sag and it will in time. At each dowel end, it will need a piller spacer between the dowel ends and the pvc inner lower end surface, enough to lift the inner dowel center ring spacer to contact the upper inner pvc center length surface, glue as req'd using 'E6000'. Now, second sag prevention option; Half split an approx pvc (380mm long) pipe and pvc cement the half pipe to the lower side of the two top left to right tubes. This is the last step in the stand frame's assemble.