I have been trying to find a way to keep my pens with my notebook. I purchased a pen loop from Midori but it turned out to be too small for my pens, I tried making my own but you can only fit one pen in it.
And then one day I was surfing Pinterest and saw something similar to this, I never pinned it so I can’t remember where it was from but I decided to make it.  I have made a similar thing before but with fabric and elastic not just elastic.
It is so simple. Just cut a piece of elastic that can go around your notebook and extra for pens. Seal the edges using a lighter, so they don’t fray, then sew it in place. I am not 100% happy with the cleanliness of the sewing, but I am pretty sure I am going to make another one and try different ways of doing it.