The firepot should be about 4" deep FOR COAL, deeper for Charcoal. If that's the depth of the inside of the brake drum, that's great. But it would probably be nice to have the flat table surface flush with the TOP edge of the brake drum.
If you need to make it deeper, you can bend a piece of flat bar into a ring that sits around the top edge. Again, I think the table should be close to the top - of the flat bar, in this case.
The drum has to hang, or be supported underneath.
But these are just improvements; suggestions. What you have looks very serviceable.
I have changes I would like to make to mine, but I also have projects I want to do, so I just keep putting them off, and my plans continue to evolve.