[norswap]

No that's not the thing about it. The thing is you put the cardboard over the book then show it to the other person.

The other person won't know where Waldo is, because the cardboard is much bigger than the book, so Waldo could be anywhere on the page.

[tango24]

That’s not what OP was referring to. OP was saying the hole still reveals a little information (Waldo’s Size/shape/posture) to the opponent.

[windows_tips]

The cardboard would need to be at least twice the dimensions. Otherwise it's possible that the book could be seen past the edges.