I've made a few traps over the years, and BY FAR the best/longest lasting/most effective that I've done has been the "spiral" type. As in, a piece of schedule 40 pipe that is ~8" in diameter (or greater) with two pieces of plate. One slit along the side of the pipe, and the two deflectors (top and bottom) funnel into said slit.
The target is held at the widest point of the "deflector funnel" to-which the bullet then enters said slit at the top'ish surface of the pipe then the bullet just rides the inside of the pipe around and around until it's energy is released.
So VERY much easier to draw on a paper than it is to describe on a keyboard. But I hope you get the 'jist of what I'm attempting to relate. After many years of use, and after MANY firings, about the only thing showing any age/wear is the rust buildup simply due to time. Scale-up and increase material quality as-per your requirements. AR500 is fantastic for the two deflectors. If this is your own private trap, then you 'could' just use mild on the sides, but if this is for any type of 'public' or 'commercial' use, then you better overkill and overengineer and overthink any idiot-quest and lawyer jamboree.
Enjoy.