There are two standard hammock designs that I know of. In one, the cloth, or string, all comes to a point at either end of the hammock. Movement in the hammock is limited, and one feels a little squooshed by it all. In the other, there are two straight stretcher bars at either end to keep the hammock opened up. If you lie too close to the edge, it all turns over and you land on the ground.
This is a new idea. Stretcher bars keep it open, but they are allowed to bend, which reshapes the hammock. You can sit sideways in the hammock comfortably and you don't fall out if you lie at the edge. If you lie diagonally in the hammock, you can stretch out pretty flat.
For flexible stretcher bars I used 3/4 inch PVC pipe, with the rope going through the pipe. Knots where the rope enters and exits the pipe keep the rope from shifting position. With just the two ropes coming from the ends of the pipe, the pipe would continue bending until you were sitting on the ground. What prevents it from bending too much are the two ropes in the middle. Adjusting the length of those ropes lets you adjust the curve and the shape of the hammock.
This hammock is made out of heavy nylon upholstery canvas that I got on sale once. The canvas dimensions are 56 inches wide by 8 feet long. The stretcher bars are only 28 inches, resulting in a bunching up of the cloth on them. I had to do that because at full width, the hammock took up too much space in my living room. The hammock is probably about 20 years old already. Nylon lasts a long time if you keep it out of the sun. This is an indoor hammock, sort of taking the place of a living room couch.