Here is a modification of brendandawes Field Note Clip. The pencil holder section was made smaller to fit what I believe is a standard pencil. The clip thickness is also reduced to the bare minimum. I made it to fit my daughter's sling so she can carry a pencil to class. I suspect it will also fit cardboard notebook covers as well. There is the letter "I' on one side and a smile emoticon on the other.
Change or remove the letter "I" and the ":)" as you wish in the SCAD file. Adjust pencil diameter if needed. Print and enjoy.