Building a backstop

Built my back stop out of a 3'X3' piece of 1/2" plywood with two wings for support, on the back of the plywood I attached two thin layers of galvanized steel plates, at home my range is max 30yards, so far I have missed my pellet trap target and hit the backstop only once by accident, the 34ft/lbs .22 pellet went through the wood but prevented from going all the way out by the steel plate