Update - due to the rounds making tunnels through the sand and then being able to hit the metal plate I did start to notice some very slight dimples in it. I think it would have taken MANY to penetrate it (if ever), but I went ahead and found another welding shop that had a piece of 1/4" scrap that was the perfect size to get 2 - 24"X24" sections out of. I went ahead and had them cut it in half and bought both pieces so I can make my dad the same exact trap. I haven't shot into it yet but am 100% sure I won't have an issue. HUGE difference holding a piece of 16 gauge vs. 1/4" in your hand. That 1/4" is heavy! Here are some not so exciting pictures, but you can see the plate stuck in there...
