diodrio Pen Holder, Pencil Holder, Pen Sleeve Case for Hard Cover Journals, Notebooks, Books, Binders, Hold Multi Pens Pencils Rulers Durable Fabrics, Detachable, Elastic Band. Black.

Here's the Diodrio Amazon store page for their pen holders: https://www.amazon.com/stores/page/23D6F554-ADCA-47D3-9F73-DA15389D4E31?ingress=0& Exact item: https://www.amazon.com/Journals-Notebooks-Pencils-Durable-Detachable/dp/B07V9G3M3P It should be noted that it's flagged as a frequently returned item. I'm too lazy to look at the reviews right now.

https://a.co/d/081R0kGz