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I suspect Alex is talking from experience whereas I'm using caution and overkill. If you've already ordered the steel I'm sure it will be fine - at worst you'll need a lighter packed lunch.
Attached - plan C which avoids drilling through tubing or trying to join parts with significantly different sections, but will need use of a lathe - there must be someone on here and local to you? If not, I could put something in the post.
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I won't be putting significant loads on for sure. I think Soma use thicker tubing but it has a stated load of only(!) 30lb. I'd not be putting much more than a change of clothes, lock and a packed lunch on there.
Alex Wetmore again has some good opinions. He seems to reckon 10mm is fine for a "heavy duty" porteur rack, going down to 8mm and even 6mm for smaller or lighter duty racks.