The limiting issue for your kettle is the proximity of the fire to the grill, that metal box is intended to block direct heat waves and divert them for a more indirect path. If you look at the Weber Smokey Mountain smoker, it's effectively your kettle with a couple added sections in the middle to make room for the water bowl (intended to add moisture and act as a baffle) and distance the meat product from the heat. That tin baffle (or some of the ceramic alternatives) probably would get you there to a degree, but you'd probably be limited to small batches and maybe extra coal/temp management.