I drilled holes with a step bit. Bunch of them. One grate sits perfectly in the tamale steamer location. I put the top grate five inches down from the lip. Installed a thermometer 6 inches down for ease of grate removal. Make sure to use only three stainless screws for the top grate support about 15" apart. 4 support screws will inevitably be uneven.
Primo XL, Pit Barrel Cooker, Tiernan SOB, Mini WSM, Big Green Egg Mini via Craigslist, 5 others, Black Thermapen