Ropes Direct are able to make bespoke ropes for your own wooden swing seats as seen in this picture.
For this particular project we used 12mm synthetic hemp rope with soft splices at all the ends and 8mm metal quick links to attach it to the eye bolts already on the seat. Not quite so visible is the connection at the top, so here's the picture we sent through to our customer when discussing the design. By using this method our customer is able to adjust the lengths at the front and back to ensure the seat sits level. We just had to factor in the amount of rope used up by this knot.