I have used quiet pellet traps for at least 10-14 years. My first one still works.
I build them with the following materials.
1-Cheap steel junction box from Home Depot or whatever 18-25 bucks
10-15 bars of duct seal (Also know as monkey shi@^) It comes in 1 pound blocks that look a bit like C4. 2-4 bucks a block. Its going to be in the electrical section of Home Depot or Lowes etc. Its heavy stuff once packed in.
1-Gate handle or something to that effect 4 bucks or so.
2 Binder clips Negligible cost
A few screws
A shoebox top or some other tray to catch the paper rips, pellets that tumble out of the seal etc.
Gut the box, drill and mount handle at top. Drill and mount binder clips at corners. Mold in 10-15 pounds of duct seal completely covering the inside back of the box. A layer and a half to 2 layers thick or so.
I have one with a lot of duct seal in it that will stop a .22 long rifle. Basically these will stop damn near any air rifle/pistol and if the round does make it through the duct seal (which it likely never will) it still needs the energy to punch the sheet metal......which it won't have.
These can be repaired easily with more duct seal or remolding of whats in it.
My first about 10-14 years old. I have just retired it to second tier. Its a small one packed with way more duct seal then necessary. That duct seal is the same stuff from when I first built it and it works as well today as day 1.
My newest. This one hold full 8.5x11 sized targets and has less duct seal making it lighter. I learned I didn't need as much as my first outing.
I actually just built a third to use in another location. These have all worked FANTASTICALLY WELL. The duct seal never loses it plyability at least in my indoor or garage use over the years. Leaving fully exposed to the environments might degrade it so YMMV.
Chris