Back in the day I beat MGS2 and MGS3 on Extreme. The box shouldn’t be your plan for sneaking past any guards. It’s for situations where you are caught out without any cover and you need to hide. Pop in to it right as they are about to round the corner. Pop out and move on once they are out of sight. The box is a crutch. You can really abuse it in MGS1, but it’s usually easier and faster to just run around the guards.