I used 2x4's to make a rectangular frame. I cut two channels in the inside of the 2x4's before assembling the frame. It has a plywood back and the top is hinged and locks down on both sides. I cast a sheet of lead about 3/8'' thick that slid into the rear channel. It can be raised out of the channel and out of the frame. The first channel holds my cardboard that I staple targets to. I put a leather strap on top to carry the trap by.
The pellets don't make much noise when they impact the lead sheet after passing through the cardboard. Sometimes they stick to the lead sheet, sometimes they just fall off. Then sometimes they begin to build up in clusters and eventually the clusters just fall off into the bottom of the target box.
I used a Daisy pump up model, a Beeman R-7 and an RWS model 48 with this pellet trap and it worked fine with all three of them.
I used a pure lead sheet in my trap. It would begin to bow after many hits, but it was a simple matter to flatten it back out occasionally. I cast it in a wooden mold I made. Wheel weight lead might work better because it could be hardened to resist bowing.
I used a 4'x4' of sheet aluminum set against my picnic table in the back yard. It worked fine to stop pellets. They'd just flatten and fall straight to the ground, but it was noisy..........Mike