This is a paper clip with a rectangular hole for displaying the selection. I recommend printing the entire clip from PETG so that the clip is flexible enough, but you can also use other filaments.

Size of the entire model: 17x10x1 mm (can be increased as needed)
Window size: 15x8 mm (can also be increased...)

I have two options, either the protrusion resting on the paper or other material has a height of 1 mm and the other one which has a size of only 0.5 mm. I like the second one better because it doesn't slip, but the first one also has certain advantages. Both models are available for download.