I'm not familiar with 'natural'. It has to be tanned. There are only two processes that I am aware of:
veg tanned (toolable, workable, moldable, dyeable)
Chrome tanned (drum dyed during the process)
I agree with 7-8 ounce, but it needs to be loose, not stiff.
Tandy is cheap but I have not had much luck with them in understanding my requests. Try Siegel of California. Ask for a sample so you can feel it, tug on it, etc. It's not just the look. I used 7-8 ounce standard oakside (oak bark veg tanned) and it looked perfect. It did not move the way I wanted it to, though.
OK - As for the progression, I have a pattern that is easily adjusted to the correct number of blocks with the same height and progressive width. 16 x 12. No problem. You basically have to figure out the angle that it falls and work from that. I think it was between 1 and 2 degrees for my riginal pattern.