Steve-what's the material you put in the pellet stop box you showed me? Duct seal or something like it?
My youngest shoots her feinwerkbau air rifle in our basement into a pellet traps I she can practice her 10m standing event. Her rifle shoots at 700 fps and as you can see, the pellet barely penetrates the material
This trap is made by archer air guns and takes a target available from pyramid air. The target is a bit wider than most targets. National target makes then and uses the same bullseye dimensions as the competition targets; just puts them on wider paper at Archers request.
The pellet trap is made of wood and has 12 pounds of electricians duct seal (aka monkey poop) in the back to absorb the pellets energy.
I put a double layer underneath the sighter bulls eyes and it gets daily use with out issue. At some point, I'll remove/replace the duct seal and she will keep on practicing.