Try building a heavy wood backing to slow the bullet and use a corrugated plastic target backup set a few inches in front. The plastic will hold the target steady and is sort of self-healing, while the wood will slow the bullet without all of the back-splatter.
Rebuilt it so the backplate is at a slitgh angle top to bottom. This will divert the slug downward towards the ground instead of splattering.
I'm surprised you haven't hit with bullets or bulltet fragments with the straight flat back.
__________________ If ya don't know where I've been and ya don't know where I'm going, your opinions of me don't count.