For decades we’ve used light bulbs to prevent water tanks from freezing. Enclosing the barrel in some manner is step one. Low es, home dep o etc have water tank insulation blankets. Something just as good (actually better) (for me its free) is using hay, square bales.
1) Just make a 4 sided wall around the water drums with the hay. Cover them with empty paper feed sacks (insulating property) then cover with something water proof like a small tarp. It helps to use something like an empty milk/water jug placed just underneath the center of the tarp, on top of the barrels. This forms a peak so rain will shed off.
2) Then use an old fashioned 100 watt incandescent light bulb and make a simple drop light. Hang the bulb in the air space around the barrels.
3) In effect you are creating a blanket of warm air around the barrels. The hay insulates and traps the air, the tarp covers the top and traps the air, you don’t want any air flow. The idea is to warm the few cubic feet of air in contact the barrels.
You’d be amazed how warm a single light bulb will keep the air. Even on nights below 20 degrees outside the air inside the enclosure (around the barrels) will stay above 50. After a few weeks the water in the barrels will stabilize to around the same temperature.
The best way I found was use hay bales plus automotive heat control mats. Most auto parts stores have it. I got mine from Summit Racing, back then a 10ftx5ft roll was about $20. These heat insulation mats have a special foil that reflects radiant heat. So any radiant heat coming from the water is reflected back into the water. Any heat from the bulb is reflected back into the water. This insulation keeps the enclosure 10+ degrees warmer, than not using it.
I cover the inside of the hay wall with the heat mat, not the barrels.
https://www.summitracing.com/parts/the-14620
Automotive heat insulation has 101 uses… Through the years I’ve used it for dozens of things which have nothing to do with a car. I still have one unused roll up in the shop.