Yes, the back boards are fairly easy to replace. My design is such that they were the last components to be installed, and they are just held on with screws anyway.
As you'll get to see in the third video, there is a pile of wood immediately behind the target area (in fact, I used to have a piece of plywood nailed directly to the wood pile), as well as a very large pine tree. The forest is also dense enough that no bullet will go very far before running into a tree, and there is a stone wall marking the property line only a few metres beyond.