I would love to make a bridge hammock, but the curves would destroy me.
Poster board, String, Push Pins, tape.
Cut poster board into about 10" strips and tape together the length of hammock.
Pin poster board to wall and place a push pin on each end the length you want the hammock
Attach string to push pins until it drops to 6" in center.
Take marker and make dots along the length of string
Cut out template
lay template on fabric, mark, cut.
If you mess up it is only cardboard and if you make another bridge you save a bunch of time.
You could fold your fabric in half and only use half the template to mark and cut like Grizz suggested