Unfold the box so you have two wedges.
Good job so far. Question: What are you scanning?
If the thing you are scanning is very thin, like a magazine or composition book or lab notebook, leave the two wedges connected. Same if it is a spiral-bound notebook. These things all work well without further cutting. If the diagonal you cut across was not square, however, you might want to cut anyway. Like in our case, because the angles weren't equal, we had to flip one of the wedges around.