It's a sheet of steel as expected - Spartan Armor Systems AR550 Steel Core Body Armor, 10x12, Set Of Two

I got the "Spartan Armor Systems AR550 Steel Core Body Armor Shooters Cut Set Of Two, Single Curve, Full Coat, Black, 10x12, SAS-AR5501012-FC" and put them in a 5.11 TacTec Plate Carrier. They fit great and hold securely. I put a few taco mag carriers on the front of the plate carrier and they actually are easier to use with the steel plates inside because the steel holds everything rigidly in place.

I have no idea how the carriers would perform if shot, but they do everything else I would expect from a sheet of steel.