Steel would be heeeeeeeeavy...
Most Fett costumers use sintra or styrene. There are some builders that buy fiberglass Fett armor which is a lot better, but obviously not easy to make if you aren't crafty like that.
I've also seen armor made from car hoods. Pretty nice, but has a lot of weight on you costume's soft parts.
If you use sheet metal, be sure to buff the edges out real good so you don't have a bunch of sharp edges.