advice for cantilever on temporary, free-standing wood structure
If it were me, I would erect two posts a fair distance apart at the edges of the yard and run a clothesline or other sturdy line across from their tops.

The posts can be put in holes dug into the ground for semi/permanent use, and/or staked with ties.  I would not attempt to make a base to hold the posts up because the leverage of a top line is so high there is always the potential of pulling the posts down.