Step 1: The first rib
The first step was to make a semi-circle from 1/8 inch rod. (red arrow) I scribed an arc onto a piece of scrap plywood and sawed it to make a bending form. I intentionally made the arc with a radius smaller than the final arc because I knew the steel would spring back. I should have made the radius about half or two-thirds the final desired radius because it sprang back considerably, and I had to finish forming the arc by hand.
The original black portion of the plant hanger is coated with a rubberized covering. I had to file through it to get to some bare metal for welding. I clamped the rod arc in place and welded it with a flux core wire feed welder. Flux core wire can dig through dirt and contamination a MIG welder cannot.