I went to a local steel fabricator to get a pizza steel cut and while I was waiting, I noticed a bin of scrap steel of assorted dimensions, marked down quite a bit. I had them cut my steel from a 1/4 inch (6.35 mm) piece. It weighs close to 30 lbs and you would not want kids or pets around your feet when you move it, but it works very well. I had it cut rectangular, but if I had it to do again, I'd have the corners a little rounded.
HC