Don't remember which magazine, but years ago Farm Journal or Successful farming, who both have a "home made and handy" section gave a good suggestion on burn barrels I used for years.

The trick was to cut three or four triangles in the bottom of the barrel, about an inch or so above the bottom.  I used a pneumatic muffler tool for this, but one could also use a gas ax.  Push the tops of the triangles in, then cut to top out of the barrel.  The top drops to the bottom of the barrel, making a bottom.  All this give air all around the burning contents, and it burns much better and lasts quite a while.

I did this for years before finding a used 500 water tank, cut that in half and it worked great for a long time.