George: There are two reasons to use the SJS bottom as the top lid. The first is for more space, as you can gain 4" additional space, since it is curved, rather than flat. If building a Mini with two cooking grates, allows more cooking space on both racks. It also allows one to mount the top rack at the very top of the barrel, easier to remove items like large chickens. The only other reason I can think of is cosmetic, makes the Mini look like a WSM. My go to models are a single grate model with a 5.5"barrel, made of roof flashing, and a two grate model, made of a tamale steamer with the rim cut off, to allow the modified SJS bottom to slip over the barrel, allowing a tight fit. Whether or not it is worth the effort is debatable, but I am retired, with too much time on my hands.
LDW