Breastmilk will keep at room temp 8-10 hours.
Frozen I guess would be good insurance. I might pack in bottles, leaving room at the top for air pressure (I like Avent bottles) and place them in ziplocks. You can freeze right in the Avent bottles. If you have room in your carryon, that might be a good idea or else risk airport baggage handling.
For a couple of hours though - maybe you could do without bottles? She may cry but she may cry with bottle anyway. And if she's sleeping. . .Maybe a pacifier may be substituted? If it's just a couple of hours. . .I'm sure she won't starve.