What size box section should I use?

I've built a timber play equipment / cubby house for my daughter, and I want to add a swing to it, so I'm coming for advice for the best material/design to use.
My welding skills are nowhere near good enough, but I have a couple of mates who can do it for me - I just need to tell them what I want.

The basic requirement calls for a horizontal beam to extend out about 1100mm or thereabouts from the post. I've attached a couple of images to help give context to my description. The section of...

Source: http://www.woodworkforums.com/f65/what-size-box-section-should-i-use-148251/

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