I would prefer to be able to easy retrieve the lead from the box, so at the moment i use a cardboard box with some clothe in it i have outgrown, there also is a piece of steel at the back of the box, and it is easy to hear if you "drill" a place in the cloth you have drilled before, and this work okay and are okay quiet in my apartment living room.
The box are additionally backed by a large cardboard box ( 3 x 3 x 1 feet ) that are filled with cardboard, and if thats not enough i do have my Bose 601 speakers between all that and the living room wall.
I am still somewhat torn on what to do with the pellet trap i will make for our 50 M range, i do know i want a angled steel plate to take the brunt of the hit and deflect the lead down, but then down there i cant figure out if i will use a layer of sand to catch the lead, or go for rubber mulch or something else .
It dont have to be low noise at my friends place, though if we get our hands on a thin steel plate i am going to back it with something to dampen the impact of the lead on it.
When i shoot my .177 hot ( heavy slugs fast ), i do get just past 40 ftlbs