Go to army surplus and see if they have old flak jacket. I picked one up and it stops my 350 point blank. I put part of in an old ammo can and it make a perfect trap. Nice and quiet. I hung it on 2 fishing pole holders last sat and put my targets in front of it, you could barely hear the impact. It's nice.
Or a 10 lb bucket of plumbers putty. No way you'll get through that and it's about 12 bucks.