How about something like
this? You can cut them up to whatever size you want.
I keep a few of these in my golf bag to keep my drinks and suncreens from cooking in the sun in the summer. My mom cuts some up to fit in her cosmetics bag in the summer so they don't melt or go bad. They don't last as long as the larger ice packs but I like how they are flexible and you can use them with bags that you already own.
If you are checking luggage, these can be reused/recharged by tossing them in the ice-bucket in your room overnight with some ice from the ice machine.
That looks useful but some insulation would also be needed.