Consider making your basket deeper - it looks like it is 6"deep from your diagram. More charcoal= longer cooks in case you like to cook overnight.
For my charcoal basket, I bought the expando from HD 2'x2'. Cut it in half, made a circle around a grate, to make a 12" high cylinder. The grate was attached about 4" from one end, making the basket 8" deep for holding charcoal, and the charcoal had 4" from the bottom of the drum, so I didn't have to attach any feet to the basket.
The 3rd rack for a diffuser works well for me. You can put a clay saucer or water/sand pan there and still have 2 racks to cook on.
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