You could probably come up with your own set of plans just by "reverse engineering" it.
What ever you do, go with a 3-leg design as it will be a lot more stable on rough ground. I have and have hunted with a couple of Double Bull seats (see pic) and they are light to transport but somewhat awkward if you need to change position.
It's another design that would be easy to reverse engineer if one so desired.
Marx Don't know if this helps but I use an old resin lawn chair by cutting off the legs then attach a piece of 3-4 ply wood, screw on a lazy susan bearing then attach the bearing to the bottom of a bucket. Cost about 10 bucks hear in Ont.