styrofoam cups, simple, they crumple, we've safety tested them too, at 40PSI its like getting hit by a football, but definetly not lethal. Although at 80PSI it'll crack windshields.
one CO2 cartridge isnt enough, you need atleast two, and the small bottleneck opening on the canister doesnt provide anywhere near the flow rate you'll require.
I suggest your replicate our old design out of ABS or PVC pipes. its just an air tank, connected to a sprinkler valve, connected to a 2" barrel.