Plastic Water barrel

consider a "stock tank heater" something like this if barrel is wide enough

Where do you live?

Do you have a basement?

It wouldn't do much to keep your water from freezing. A fish or livestock tank heater with a thermostat would probably be the best answer to your problem.

Take all the money spent on insulation, heat tape, electric heaters, electricity to power the heaters ... and look for a sale on big jugs of bottled water ! Store them in your heated home ... With all the money I just saved you - build a nice sturdy rack for storing the jugs. Yer welcome. John T.

Insulation slows the movement of heat. With no added heat the water will eventually freeze, but it will take longer. Once frozen, the insulation will slow the melting once the air heats.

Use the insulation but with some sort of immersion heater right in the water.

If the heat tape is below the water level in won't melt the plastic. Whether it will keep the water from freezing is another story. Around here you'd probably wind up with a large ice cube floating in a thin layer of water.

We raised a few ducks.

In the winter I put an aquarium heater in their water bowl and it did fine.

But that was Virginia. If you're in Alaska, probably not.