As I mentioned in an earlier post, I, too, had this problem with the top grate (it was ever so slightly too small, so that it would fall off one of the grate supports if it was pushed all the way to one end). After reading through this topic, as well as an earlier thread, I decided to measure my top grate; it was the correct size (I took it to Home Depot and lined it up against the stock grates for 18" grills at the store -- it was identical).
I recently came up with a fix for this problem. I think the top of my middle section of the smoker must be slightly out-of-round, which is why the grate can slip off of one set of supports (on opposite sides of the smoker), but not the other. So I inserted metal washers between two of the the grate supports and the outside wall of the smoker. With one washer on each of the two supports (at opposite sides of the smoker), that "moved" the supports out just enough to solve the problem (and at a minimal cost of $0.26).