I don't understand your math. The 1mm (the wall) was there already, so why is it being counted here? Plus, multiplying by 1 doesn't do anything? Also, the 2mm extra won't be solid plastic (they'll be solid air, since that's why we're adding the extra thickness, for the room.

If anything, the extra material for the case would be the perimeter length times the perimeter wall width times the height.