ar500 grade steel would work for just about anything. A 30-45 degree angle down (force the round into the ground) is good as long as the ground is soft enough to absorb the bullet.
As for thickness, 1/4" will handle a 22 LR from 40 yards or so on out but I'd suggest 3/8" at the least.
Pay it forward - Kids are the future.
Rifles are similar to boats and young women...there's no end to how much money you can pour into them without making them any more useful.