They lay flat in the freezer very well, which allows the milk to freeze in a rectangle shape with similar thickness. (tinyfry.com)
To make your own, all you need is an image or shape on any kind of paper, some scissors, glue, a cereal box or cardboard of similar thickness, and a hole punch. (sortacrunchy.net)
Peel the potatoes and courgette and slice in similar thickness. (wander-crush.com)