16 inch diameter and 18 inch length is just under 16 gallons. That's stuck to the side of my Brinkmann, which is also 16" diameter. 55 gallon drums are about 24" diameter; see if you can find a 25 or 35 gallon drum for the firebox.
The metal on those is quite thin, so it won't hold heat like propane tanks will. But those are dicey to cut and weld on unless you go through the 2 week neutralizing process...