Tim Watts :
The actual swing rope would be attached via 4 holes in the corner of the
seat like this:

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/ \
OOO

Would you be lashing the three logs to two lighter ones at right angles,
to keep them in one plane (g)? If so, a square lashing is what you
want, though you might find it hard to get the three logs acceptably
close to each other.

--
Mike Barnes