How to temporarily mount a large object on the side of my house?
Based on the restrictions you've presented, your post idea isn't farfetched and I don't think it will look particularly unusual, especially if you consider to paint the post on which it is mounted to match the color of the siding. From any appreciable distance, the post will mostly vanish.

On close inspection, or a second look, the post may "pop into view," but the viewer will have already experienced the primary focus of the snowflake.

If the post idea is still rejected, the hanging idea could be better implemented by attaching it by cables to the eaves, but also attaching cables to the ground, to prevent movement. You could get away with a minimum of three cables, one above and two below, or two above and one below. A padded substance behind the snowflake will improve circumstances, as well. Use enough padding to push the cables all the tighter, forcing a more secure friction attachment to the wall. Paint the cables to match the siding?