You don't want to just run bare steel plates in a carrier. They need to be coated with layers of Kevlar/resin (although some guys have been having theirs Rhino Lined). That's what is going to help prevent spall/fragmentation. If you're a DIY guy, you can go that route, but by the time you factor in cost, time, hassle (laying up Kevlar/resin is so much fun), and operator headspace and timing, you're better off just buying a set of commercial plates. While something is better than nothing, do you really want to cheap out on a piece of gear that you're going to bet your ass on?
They are also meant to be worn in conjunction with soft armor. Not as stand alone units.
This is what happens when rounds impact bare steel plates. Imagine what happens to your face, arms, and thighs you if you're wearing these:
http://www.youtube.com/watch?feature...&v=SCn8KOoBNxs