If that was the only thermo I had, I think I would put it in the lid. If it was a flat lid I would put it in the middle of my circle of holes. If you have a chimney, I would put the thermo midways between the chimney and the furthest edge of the lid.
Whatever you decide, all you have to do is clock it. Put an oven thermometer on the grate in the middle and write down what the outer one is. Then open the lid and check the one on the grate and write that down beside it. If you are going to use two grates, put an oven thermometer on each. On second thought, if using two grates, put the thermometer in between them on the side.
When you season the drum, do this every hour to give the drum time to adjust. Do it every so often afterward during a cook and anytime you calibrate your thermometers if you want to be really sure. Do this no matter where you put the outboard thermometer or what length the probe. As long as you do this, no matter the difference in your outboard thermometer and actual center of grate temps, you can accurately figure the temp inside without opening the lid.
You can use any type of thermo you want, and you'll have the added benefit of no one being able to cook on your cooker quite as successfully as you do.
Hot Rod Hog Cookers Association, Charter Member, President