Need advice for a backpacking chair: fabric

I don't have answers for you—but my wife will sign up to be your first customer!

Landscaping fabric? Very light, strong, and easy to cut and either sew or glue. Cheap, too, if you want a lot to play with for prototyping.

There are various weights out there, thick shiny 'professional' grade down to no-name hardware store brands.

this stool has, I believe, something like a 400 D fabric and it is listed at 10 oz.
I got mine below 9 oz with some scissor work.

Might get some ideas from it??

I like your specs for intended load capacity!

I've noted that the new versions of the REI 3-leg stool now use a burly mesh in the center of the seat and weigh very slightly less than the older, rip-stop versions. A few of my Scouts have them and I think the mesh is great for those times when the stool gets rained on…plus it saves a gram or two. So, my suggestion is that you go check out those stools to see whether that style of construction and materials will, uh…"mesh" with your needs.

Cheers.