But does the foam collapse when pressured...? I don't know of any approaching a lateral-force strength of concrete, despite claims. If it could give way whatsoever, once there would become any freeplay it would soon be all over. Have been any cases of failed foam posts?
By flattening out the base of the round post it becomes anti-rotational in cured cement. Also it's anti-rotational when directly driven into soil. Commercial direct-drive steel fence posts are the most anti-rotational of all, usually being of both 3-D cross-section and having a large flat tab attached near the base. If you could drive this and properly attach a regular dish mount to it, it should hold quite well.
View attachment 186005
View attachment 186006