We built our cargo rack with 1" square tube, but it's supported on both sides. Stacked tubes should be fine, but I'd for sure add gussets.
Our rack flexes noticeably (if you're paying attention) when walking at the edges, and it's about 2' from edge to support.
I don't see why 2x6's wouldn't work, but I'd worry about them degrading quickly. However, it would look cool as ****.
I'd start with pressure-treated lumber, do all bolt-through bracketed construction, and then paint on a thick layer of used motor oil. Then re-oil seasonally, in the summer. I would not paint- then it'd be real hard to see cracks
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