The easiest way to use reusable pens and penfill cartridges.
Using this type of thermos:
http://www.amazon.co.uk/STAINLESS-T...qid=1445952310&sr=8-7&keywords=thermos+flasks
You can get around 20 3ml penfill cartidges into the flask in their plastic bubbles and leave the boxes behind. Chill the flask with iced water before using it. That's the approach I took when going to Australia. It's also more robust than the Frio wallets.
In all fairness, you probably don't need it on the 24 hour flight. I had it because I was back packing and knew there were occasions where my stuff might get a bit warm.