Steel skeleton
I mostly used 3/8" rebar and 1/8" steel rod from scraps, or Home Depot. To make the tree climbable, the frame needed to be pretty strong. So I over-engineered it to easily support 3 adults, even though it would only ever be used by little kids. As I worked, I'd hang on the different parts, testing for strength.
I for one am glad I saw this ..Be it "again??" or not...