These are the back up plates on my car as standard Matty.
I notice that those backing "plates" are very, very thin and do not have rounded corners. I guess when they are so thin, there is no worry about the corners producing stress with such a large patch compared to the thickness. But with that, it seems the size of the plate (i.e. thin sheet) is not actually the size of stress relieve you get. It looks you could bend it easily with a finger by pulling at a corner if it was not glued on (as I suspect it is).
