Just put the box at the right place. And launch the right spell when you are well placed. I won't be more specific. I do not want to spoil you the pleasure to find what to do and finish the game :)
Thanks again for your help Fred - made it through with your help!! I was trying to use the box from the ITEMS screen ... but that's not right. Clicking when positioned at the right spot *when facing toward the top of screen* was what finally got me past being stuck.