While at the Deer and Turkey Expo, I came across the guy selling the sit drag hunting seat. It looked like exactly the sort of portable seat I was looking for but because I'm the anti consumer and would rather make things my self, I did just that. For an earlier project, I had purchased 300 yards of Automotive seat belt material that is rated for 3,500 Lbs tensile. (so it should hold up my considerable rear end) For the seat portion I used heavy black canvas.
I sewed the parts together on a 1950's vintage Singer slant line that I have used for other projects. The thread is used for upholstery. It thick and strong. Wrapped around each hand, I am unable to break it. I used more stitching that I probably needed but it made for a strong joint. If your looking for a light weight portable seat, this may be just the product for you.
You can see how the sling seat is used in the videos from the sit drag site.