I came up with these on accident creating some parts for another project. They hold very securely.
Note there are 4 files.

Pen Holder with mounting holes.

Pen Holder without mounting holes. (If you plan to attach with adhesive).

Pencil Holder with mounting holes.

Pencil Holder without mounting holes. (If you plan to attach with adhesive).

For your reference, the Pencil Holders are 10% smaller than the Pen Holder in all dimensions.

The Pencil Holder without holes in the photos was printed in 8 min. using 0.3 layer height 100mm/s 240° with CHEP's Experimental Settings.

I recommend using PETG with 10% infill. If using PLA, 20% infill.

*Note that some printers may require you to shorten the file name for the printer to identify the file on the sd card.