Use what we sometimes here in the Midwest call "screenings" (FA5). They are a pretty fine crushed limestone that'll settle or can be packed down and become almost concrete like but will still drain. I would make sure the hole is big enough to fit the end of a 2x4 all around the post. That'll give you room to put enough screenings into the hole to be useful and to use the 2x4 to pack them tight.
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