The Built a Bunker Backstop article.
From my experience sand in plastic bags that get punctured generally runs out of the holes, after a shooting a ten-shot group on all of the four targets, the next thing you would require in missing illustration No.13 is a shove to put the sand back in the plastic bag after you have patch up the holes.
And the raw edges of the front of the slabs should be covered by a material that doesn’t cause ricochets.
And if you have young children you need to keep them well way from this structure as it has the potential to cause them serious injury if they play on or even in it.