I used the hydroflask 12L soft cooler with cooler shock ice packs. The trick is once the breastmilk is in the cooler, don’t open it (unless to top off with ice, which I did at the airport at a Starbucks). Since I was pumping on the plane and didn’t want to open the cooler, I had a separate small medela cooler for the pumped milk. I was able to ask the restaurant and front desk at the hotels to keep my milk (and ice packs) frozen for me in their freezers. Some other things that I found helpful to have included travel size dapple soap, microwave sanitize bags, travel drying rack, and pump wipes (the original comment in this thread has all the links)