I use my freezer a lot and use plastic boxes to freeze the food in. The only problem is when I buy new boxes as the old ones wear out, the lids aren't always interchangeable! I want a way of marking them (so that I can match up the tops and bottoms that fit together) that will survive going through the dishwasher. At the moment I use a permanent marker pen but that only lasts through a couple of washes. Any thoughts?