I have lots of steel targets made from 3/8” mild steel that I got from a local steel supplier. It was in plates that had been cut incorrectly for some project so it was put in the scrap pile and sold cheap. So check around and you might find a good buy.
Those critter targets have been pounded with everything from small bore m/l guns to .45-110 Sharps for the past 20+ years. Some of the buffalo in our herd of 50 are bowed a bit, even at 375 yds from the 500 gr bullets, but I just turn them around and eventually they get pounded back. The close up targets, 25-100 yds, get used for all caliber m/l & lever guns and will get a bend here and there but a big hammer takes care of it. Other than that, once in awhile a foot or base gets knocked off but can quickly be welded back on.
Ted K