Styrofoam coolers in a wax lined box (can buy fish boxes out there many places) is a good way to bring back fish....the coolers are light and won't take up much of your 50lbs /bag limit.
The fish cannot have any ice packed with it....must have freezer packs for transport on the plane....be prepared to show you have no ice packed with the fish, be prepared to dump ice in the parking lot
Speaking from experience....
LC