Instead of a curved tree contact surface, why not make a V bracket or Batwing style surface that can mate with any size tree? I also think that weight will add up quickly as the size of the platform grows. Best to keep it small for mobility, but wide enough to put two feet on, at least for the center step. If it's not long enough of a platform surface for an entire foot, it might get uncomfortable as it forces the toes to be at the tree trunk but heels hanging off the edge. If the platform is held a little further out from the tree, as in an angled surface mounted to the top of a climbing stick, you could use a shorter surface and center your weight on the surface with a little toe hanging over and a little heel hanging over.