We used to use plumbers putty as a back stop for pellet shooting. When we once did "testing" (really just goofing around), we used two sticks of equal thickness and a rolling pin to achieve a consistent thickness for the test medium. Same way that you grade gravel for interlocking brick.
We did find that you needed "extra" medium so that there would be enough give in the medium, otherwise an adjacent shot would tend to deform things too much. Maybe not a problem with yours as essentially a backer was used. (we used 2" thick free standing chunks)
It would be most interesting to repeat these test at a further distance to demonstrate the momentum effect of heavier bbs. 2m vs. length of basement.