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I use an AMNPS to cold smoke in my 22.5" WSM, as for add on.... none are really needed. But a lot of folks here like to use some sort of temp. controll blower (iQue, ect.). However even a stock WSM will run steady and is very easy to use as is.
The only must have mod in my book is either eyelets or a slot for temp. probe wires. If you look bellow my signature line there is a link to the thread covering making eyelts and there are some comments in it about the slots.