Will the top of the pile (excluding the hook) be flush with the top of the box and the sand? Or can we have the pile height above the box?

The top of the pile must be flush with the top of the box and sand. That means, while the wooden dowel you will purchase from the hardware store may be 60 cm (24 in) or longer, you will need to cut the pile (dowel) so the height of the pile matches the height of the box which is 146.3 cm (18.25 in) without the hook attached. The hook attached to the top end of the pile will extend above the surface of the backfilled sand.