Posted: Sep 16, 2016 - 03:34 PM
I seriously doubt that you'll find a design for one online.... at least not a decent one; this is what the sail makers are really selling, their shape/design.
Also, given the cost of materials involved (N17 spi = 19.5m2), assume 20% cloth waste, so would need ~17 yards of fabric. 3/4oz runs about 11.75/yd online, so there's ~$250 with shipping. Tape for edges, grommets, nylon webbing, thread, seam tape, figure another ~$100? Then figure your time... a LOT of time.... only to come out with a questionable product. Lofts use plotters to draw/cut their panels out, getting them straight and in the correct location isn't a trivial task.
There are PLENTY of good used and new spinnakers online for $200-800 from reputable sail lofts that will have a good shape; doesn't seem to make a lot of sense to try to make one from scratch if you don't have the equipment or experience.