I don't really see the point of using anything in the pan with a Stoker or Guru. Putting something in the pan is for when you need a heat sink. Stokers and Gurus should not require one. I haven't used my Guru for a long time but I never put anything in the pan. The Guru controlled the temp.
Fitting the brisket: Just jam the ends between the handles of the upper grate. Bow the middle of the roast up, if needed; it will shrink as it cooks. I usually foil the thin end of the flat when I do this so it doesn't get buffeted by the heat rising from below. You can see a pic
here, some way down the thread. That thread describes a brisket cook.
This one describes another. As you'll see, I do briskets at high heat. It's my preference but doesn't mean it's a 'better' way. Better for me, imo. It's a different approach. Doubtless there will be more than one brisket cook in your future so you can try different methods and recipes and see what you prefer.
I haven't made a marinade for a brisket in a while. When I do it is usually buttermilk-based, with onion, garlic and an herb or two, salt, perhaps a chile, pureed in. Most people, probably, don't marinate, but I do like them marinated from time to time. Most just use a dry rub; I like a dry over paste rub.
