These work as well, and are like six dollars from amazon - (10 pack of adhesive pen holders, including some wire and some fabric)

https://www.amazon.com/XIWUMOER-Adhesive-Stainless-Notebooks-Calendars/dp/B0B19V5QYL/

the clips are really useful (they fit tight) and the elastic is self-adhesive, and if the elastic is too "loose" you can tie a tiny knot in it with thread to make it even tighter.

In the long run however, I just ended up getting a padfolio that zipped with the pen inside -

https://www.etsy.com/listing/1206035651/supernote-a6x-hardcover-book-looking

etsy is your friend for these, though i have to admit i'm partial to the above because this one actually zips "shut"