I was surprised at how little trouble I had with this one. I was grateful for the abundance of reminders about the new keys for Blender 2.9 as well.
Notes I took: Merge Vertices by Distance is the same as removing double vertices – Under Mesh option tab or M on the keyboard while in editing mode (tab on the keyboard).
I also removed the inner face. I think this shape would make a lovely birdcage scene.