TargetTalk

Rover: After 40K shots of say .177 8.44gr pellets you could open a leadmine :-) There would be something like 48 pounds of lead in that cardbord box :-)

If you have 48 pounds of lead maybe you could make a lead back stop?

If you leave the box alone, that's EXACTLY what you have.

You could probably sell the box to bullet casters.

Here is mine. It is 3' x 3' 10 gauge steel faced with 3/8" of rubber matting used in horse trailers. The steel is bent 90 degrees to fit in the corner. The steel costs $75 from the fabricator. The rubber sheet was about $20. I have no concerns when I introduce beginners to the sport. Of course they start at 3 M and work their way back as they keep them in the scoring rings. They also start in the seated supported position.
The benefit of overkill is that I can change traps and the backstop would still be effective for .22 LR. 10 gauge steel wouldn't be used for a backstop on a commercial range but it is used for overhead baffles on a .22 LR range. It will work fine for the occasional strike.
The targets that are resting on the counter are inventory. They are not shot at when they are stacked up there.
I tested the cardboard box full of cardboard. The P11 would penetrate 13 layers of new (unholed) cardboard.
Rubber granules traps work great also but will leak rubber when they get shot up.