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I'll build one and show you...

stay tuned.

RAW, A spreader bar could be hung vertically. Then your hammock can attach to the the spreader, reducing the lateral stresses. Bracing can also be installed to reduce stress/torque.

I guess for...

Purlins can be extremely strong..or else they wouldn't use them for holding up the roof.
I did major construction for years and I can tell you from experience those purlins are strong.
Look inside...