I’m one of those people who wants all my BuJo stuff together. Therefore, I do not like carrying a separate pen case. I found the Quiver pen holder on Amazon and haven’t looked back.
I like to carry 2 pens with me at all times. Usually, I just put both of them under the elastic band. The problem with that is whenever I want to just look something up in my BuJo, I have to remove the pens because they fall out when I move the elastic. This pen holder really helped with that. The part that actually holds the pens is pretty snug… if I turn my BuJo upside-down the pens do not fall out (although mine are clipped to it. Not sure about pens with no clip.). The only thing I don’t like is inside the notebook the back of the pen holder leaves a black imprint on the page. Not too big of a deal for me, though.
Here’s the Amazon link if you’re interested. (Quiver is just one brand, I’ve seen a lot of similar pen holders on Etsy, too.)