"Magic Bowl" effect with an aluminum foil roasting pan
Covering loaves during the first third to half of the bake is one way of achieving a humid environment in a home oven, Its purpose is to approximate the effects achieved by injecting steam into commercial ovens. It enhances oven spring and the spreading of cuts (bloom) in the loaves. This technique has been discussed extensively and repeatedly on TFL.
Various members have used pyrex bowls (with the risk of shattering), stainless steel bowls, larger roaster covers and ceramic covers such as "La Cloche." Each has its advantages and limitations. Most bakers want to have covers for both boules and long loaves such as baguettes and bâtards. Oven size and baking stone size present limitations to the feasible options.
After having had success baking boules under a stainless steel bowl, I wanted to use a cover for baking long loaves. I used the cover and the base from a large enameled steel roaster a couple of times. Its length was just right for my baking stone, but it was a bit too narrow to comfortably accommodate two bâtards. I had loaves stick to the sides of the roaster a couple of times, damaging the crust. Looking for alternatives, I found a couple of large, light-weight aluminum roasting pans in my pantry. They are 15.75"x11.25"x3". My stone is 16"x22", so they fit on the stone well.
They don't have quite as good a seal with the surface of the stone as more precisely made alternatives, but the results of using them is pretty close to that achieved with other covers. On the positive side: They are meant to be disposable (although I use them over and over again), and they are very inexpensive. Also, they come in a wide variety of sizes and shapes.
The brand I have is called "EZ-Foil." Here is a link to the company's web site:
The online vendors I found only sell these in large quantities. I got mine at a local hardware and housewares store in a package of two pans for something like $5.00. Maybe less.
I have no financial connection with this company and am not saying it's "the best." It's just the one I happen to have bought.