I don't know...maybe a 70's Weber cookbook has the answer.
I was thinking that it might fit on the tabs that the cooking grate sits on, then put the cooking grate on top, and that's what the sides on the smoker attachment are for, to hold the cooking grate just off the smoker attachment
Then it would act as a heat deflector, like the water pan in a WSM
Mind you it would be a bitch to access the fuel or add more smoke
Is there any dimensions for it in the catalog?