You can buy the whole seat, but you'll only need the edge!!
Unfortunately this video is not that exciting, but here's some options for connecting your tarp to a tree, be it with a full ridge line, a single line, or a double line leaving room for your hammock suspension.
For the most part I use a full ridge line with my tarp hung under, and the other methods I was showing I was pretty much doing for the first time, but I think they're viable options. What they lack is the mechanical advantage of the figure 9 but I'd bet they would be suitable ways to hang a tarp. So prolly nothing really new here, just putt'n it out there.
Comments and suggestions welcome. And if you've got an interesting way to hang your tarp please share.