The problem with this method is that the steel will darken and flake into your kilns creating a black soot messing up the kiln. Of course you can vacuum out your kiln but overall this is a messy option.
Hadar Jacobson, creator of a wide variety of metal clays instructs individuals to make your own firing box out of fiber boards and blanket. LINK
The downside to this is that cutting and "playing" with fiber blankets and release fibers into the air that aren't necessarily wise to breath in (that being said breathing in the carbon is a bad idea too, so remember to wear your mask!!!). Still her ideas are very creative!!!
There are a few new items on the market that may be an improvement over the above. The first is the below ceramic firing vessel (photo courtesy of Art Clay World USA) and the following link describes how to use it ArtClayWorldUSA - ceramic firing container.