I pretty much only pack fish out if there is snow available. That worked for us a couple weeks ago. Of course snow can be hard to come by in some places. I’ve had fish do fine with no extra cooling if it is not a hot day and they are wrapped in as many clothes or other insulation as possible. Old school Thermarests used to be perfect for that. Freezing a disposable water bottle full of water would be a good source of ice, but I don’t now how long it would last on a hot day.