Before we put in freeze proof tanks, we used to put metal 55 gallon barrels in water tanks and burned small wood fires in them.  We used number nine wire to anchor them in.  Sand in the bottom will help hold them down as well, but still need the wire to hold them in place.  Keep the metal lid and use a couple of wires across the top to sort of hold the lid where it should be, this helps control the amount of air the fire can get and will make the fire smolder for several hours. 

We burned a lot of cottonwood and hardwood slabs (from a sawmill)  Slabs stack tight and smolder better.   Usually could get 10 to 12 hours of open water in the tank out of each fire in 0 degree F temps.