This model is a small holder to glue onto the cover of your notebook to hold a pen.
3 sizes are available, 8, 9.5 and 11 mm depending on the diameter of your pen. Chose a diameter slightly smaller than the pen.
You can also chose between blank models or model marked by the letters: ♥ 3D
Letters are much more visible with more intense filament colors.

A customizable model (text and diameter can be personalized) is available at: https://www.thingiverse.com/thing:5028279.

It should be printed without support with 0.4mm wide layers so that the walls are 2 layers thick.