msdolittle,
The barrels seem to be deep enough to keep the water cool. If I put my hand in, I can feel the surface of the water is much warmer than if I lower it all the way to the bottom. I think the depth and volume of the water help maintain a consistent temperature for the fish so they don't get shocked at night if the air temperature drops.
The only predators I've seen in my 'hood are cats and the occasional racoon. The cats aren't a problem at all, but last summer I did have a racoon that pulled one of the water hyacinths out of a barrel one night. All the fish were okay though. I don't think the racoon could catch the small fish because of the water depth.
Sounds like one of these would work for you. My barrels are actually old whiskey barrels and have the Jack Daniels name and emblem on the bottom. I did have to wait awhile to get the alcohol smell out of one of them!
You could use anything though, like a nice large (but light) fiberglass pot.