A Beis Hamikdash Shape Puzzle

What object in the Beis Hamikdash was shaped like an octagon, made out of gold, and displayed prominently for all to see?

Two more clues for you:
• The octagon was only a part of the object.
• This object was present in the Second Beis Hamikdash but not in the First (or presumably in the Third).

Here are some of the pieces I used to build a Lego® model of this octagon:

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