I wanted to have a cap with a clip on my pencil (Edding 3000) because it was constantlly falling out of my pocket. So after a bit of modelling and testing i came up with this finished design.
It is a simple design that should hepl you to keep your Edding secured.
I only testet it on an Edding 3000

print settings:
material PLA
0,16 mm layer height
3 walls
infill 30 % (gyroid)
enable tree support