All electrical box plates should have rounded corners because electric boxes have rounded corners. I took a metal file to mine as well, just to make sure they were free of burrs. In addition, I have a very nice micrometer and checked to see if those plates were legal, which they were. I'm not about to spend 40-60 hours building armor so a martial can go "I know that suit weighs 30 lbs, but I'm going to give it a 2 pt armor rating."
So if you want to be certain your metal is going to stand up to inspection, grab either a micrometer or a piece of 18 gauge metal (to compare with), and a magnet. Aluminum is non-ferrous, so if the magnet doesn't stick to it, it isn't 3 pt armor.
When using large metal plates, you want to make sure there's no way for the plate to "twist" so that a player can land on it's edge, they need to lay flat against the body.