A couple of ideas I've wondered about inspired by some of the junk piles around the house:
-muffler: seems like the size and shape is good, and many already have end caps with holes that might be suitable as doors
-soda 'keg': Fountain drinks used to be dispensed from these 5 gallon stainless containers that are now used by homebrewers for kegging beer, you may recall seeing them in gas stations, restaurants, etc. years ago before they switched to cardboard boxes with plastic liners. They look to be a nice size, shape, etc. for a forge