There is a local pallet works that my bud with a weld shop rolled some gigantic barrel shapes for out of 3/16" plate steel and basically copied double drum burners .However they have heavy steel stiffeners along the sides because they will warp and need attention every 4-5 years .Think about it pallets ,oak .The fuel doesn't cost one red cent because it's all scrap and saw dust
.It's a pretty good sized shop too all heated by barrels ,sort of with fans of course .