you want to use Flat mapping for the image but because the front face of the box is flat to the viewer (and the texture axis) that part of the image (the center bit) can stay actual size but the two sides, being at an angle to the viewer need to be scaled thinner so it fits from the front but the flat projection makes it the correct width when viewed from the side – just have to work out the scale size based on the ratio of the size it needs to be (whatever the width of the face on the back side is) to you want (2.719)…
9517_boxmap.zip that’s wrong since I don’t know the actual width of the box but that’s the idea