in my small offset, 16" x 36", there is the fixed baffle which extends about 8" into the cook chamber, and then I have 5 - 5" x 16" x 1/4" tuning plates. Sometimes I use all 5, sometime just 3. It all depends on what's cooking. To learn how they worked, I first used all 5 with varying gaps between them. 1/8" between the first two closest to the fire box, then 1/4", 3/8" and 1/2"+- for the ones further away. The idea is to let heat rise between the plates. Push them all together and the grate temp away from the fire box will actually be hotter than next to it. Experimentation is key here. Have fun!
Oh yeah, a local metal fab guy made up my plates and I paid him with a couple racks of babybacks.