And you also achieved an "aha" effect. It was like "ah! what if this box is an wooden box" and then the puzzle was practically solved. If you wanted this then you did everything right and i was just impatiently. But if you wanna something like "the fence is wooden and also the box.. ah! then the hook should work on the box too" thought, then you just need to give the fence and the box wooden pattern.
I just replayed your game and now I saw that you explicitly say that the box is a wooden box. But for me this information got lost and a permanent (visual) wooden pattern would help, at least people like me who overlooked or just forgot this piece of information :)
However, I'm looking forward to play new levels :)