I tried safety pins while testing some ideas. They didn't hold well enough to test properly, my hammock ended up with a couple of tears, and some of the pins bent and snapped open stabbing me.
You would be better off taking the plunge and trying your ideas with full-fledged suspension and fittings. If you want to cut costs during testing, you can try less expensive/heavier materials, like sheets for quilt shells. Once you get the fit you want, you can unsew the prototype and use it as a pattern for the more expensive/lightweight material.