Having been a fabricator for over 12 years now and specializing in arc welding "exotic metals" @Chris is completely correct. Aluminum is not dense enough and much too conductive (a little over 4 times what plain carbon steel is) to work well in the oven (this is the same reason it works so well as a heatsink, low mass but high dissapation of heat) and stainless is 1. super expensive and about 4 times less conductive than regular carbon steel so it just won't pump as much heat into the food you are trying to cook it on. Cast iron could work but heavy and in that configuration it can be brittle especially when heated and reheated. 1/2" would have no noticble difference when cooking something as small as a pizza, 1/4 should be plenty. Make sure it's clean as carbon steels are coated with some sort of oil to prevent rust and they are almost all petroleum based. I use a 1/4" sheet of high carbon steel (still conductive but less prone to rust) I aquired when working on a project welding a water resivour for the local water company. It works great, way better than a pizza stone, especially for that thin flaky charred crust.
