My bow shop owner rotates his spot target "blocks" about every six months. They're AT LEAST a 4x4....and they're not in bad shape. He sells the used ones for $65.

If I didn't have a SAFE place to shoot at my house....I'd shoot elsewhere.

Your area is NOT safe (as described)....even if you HAD that large a backstop. "I" think you need to address where you're shooting.