 The pairs of holes on the far left and right hold the rails on.. 8" spacing seemed appropriate to avoid flexing of the aluminum angle. Based on this rhythm, I chose a 16" spacing for the internal ribs. These then get two bolts in the top and bottom and two bolts in the sides. The side ribs wanted to be supported often as I was only using break down fasteners (no glue). 8" spacing fit the established rhythm..
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