FRIO Cooling Wallet - Insulin Cooler for 45+ Hours of Stress Free On The Go - Blue - Individual

Get a FRIO Wallet. That is what outdoor folk use for camping and hiking. https://a.co/d/02jv046X

https://a.co/d/097Djq1eThese are great for backpacking, you can get ones large enough for as many pens as you need to bring. You just dip it in water and put them outside your pack in the mesh pocket so that there is air circulation to maintain cooling.