I used a full size Pentel R.S.V.P Ballpoint Pen, with it's clear barrel, and tied a couple of Gaucho knots and a long two-bight Turk's head knot(2 bight 19 lead twice doubled) around the ink cartridge with orange 0.9mm cord, and then reinserted the cartridge back into the pen body
I would normally have used only a single strand for all of the knot work, but if I had run the working end back underneath the long Turk's head knot to tie the last Gaucho, it would have made the knot just that much thicker, and I wouldn't have been able to fit it back into the pen. So, the end Gaucho was tied with a second strand of string. I did not measure how much cord I ended up using, as I was working from what I had left from a previous project. I tried this project first with paracord over the cartridge, then knot work over that, and unfortunately it was too thick to fit, but I'll certainly keep an eye out for a pen where that may work.
It's a snug fit as is, but as long as the knot work is tight around the cartridge and not tied into a too thick pattern, it can be fit back inside the pen body, with a little careful pushing. I'll have to try different knots or hitchings to see what other variations will and won't fit, when I get around to buying a few more pens to play around with. I have some glow-in-the-dark sewing thread that I'd thought of using, but I imagine it would make for quite a tedious knotting process with its much smaller diameter.
Various pens can still get knot work on the outside if desired, or as some folks on the Facebook Parachute Cord Crafters group have shared, can be modified to hold a short section of paracord sheath over the ink cartridge, and still fit inside the pen body. The group refers to them as 'parapens'.
It makes a nice way to use up leftover short sections of paracord and inexpensive giveaways to friends, family, and strangers that always want to 'borrow' a pen. At least one group member has put up a photo tutorial for a way to help fit the paracord inside the pen, although I just kind of push/pulled/twisted the ones I put together.
The other pens used were Bic Cristals, Pilot Better Retractable, and Pilot EasyTouch stick pens. I'm sure a visit to an office supply store would offer a few more possible knot work victims.
For a DIY type, find the right sized acrylic/plexiglass/lucite type tubing and you'd be on your way to making your own pens or clear key chain fobs/storage tube with knot work inside. Maybe tie knot work around one tube, fit that inside another, then put caps on the ends with an attachment point...