I wonder why we're not given - as we usually are - the number of cells to be highlighted. Those who have found the hidden goody will doubtless be able to advise if that information has been withheld for a thematic reason, but somehow I doubt it. I'm having difficulty resolving "word square" with symmetry in a 12x13 grid. More gazing called for. (I have the letter, and am assuming, since I can't see anything, that the message does not appear in that shape.)