Simple stick-anywhere pen holder.

Attach to anything with a command strip.

Print in TPU to make it flexible, so that it can stretch open and closed around the pen.

Pens easily snap in and out with one hand.

You can scale the model for thicker or thinner pen sizes. But this size works great on most pens.
Mount it horizontal so that smaller pencils don't slide out.
Even holds large dry erase markers.

Creality

Ender 3

No

No

.2

30% Cubic

It has a slight overhang at the top but no supports were necessary, and it turned out great.