AR500-AR550 x 3/8-1/2 inch plate should be fine, set in a downward angle for bullet deflection. Price can vary and be pricy, depending on where and how much you obtain.
"There is no hunting like the hunting of man, and those who have hunted armed men long enough and liked it, never care for anything else thereafter." - Hemingway
“The greatest ignorance is to reject something you know nothing about.”