It has to be under the wood.
I have one in my Lang & have it positioned on the side opposite the door, below the wood grate.
But I will tell you that I never use the Guru anymore on the Lang.
I can control the temps better with the dampers than the Guru can.
It wants to overshoot the temp, then it just shuts down, causing a bunch of thick white smoke.
Your going to have to be around the smoker anyway to add a split every 45 minutes or so, so why not just learn fire control.
Now it's a different story with my
WSM, I use the Guru on that every time I use it, especially overnight cooks.
Using the minnion method I can get 22 hours of continuous smoke & heat, and not vary more that 1 or 2 degrees, with out adding any wood or charcoal. It will hold any temp for as long as there is fuel to burn.
Al