If you use cartridges in reusable pens, you can keep yet-to-be-used cartridges in a small (0.5 litre) stainless steel vacuum flask, which protects insulin from being bumped etc as well as protecting against extremes of temperature, both freezing and heating. Very cheap - I've seen them recently in supermarkets for under £5. Flask needs to be empty of water when passing through airport security, then you can fill will cool tap water.
But, generally, keeping in use pen(s) and insulin out of direct sunlight, away from radiators, engines etc, wrapped with clothing, deep inside hand luggage is usually sufficient, especially if you are going to mainland Europe for 1 or 2 weeks. I've only used the flask for a month or longer in Central or South America.