I had a simple trap with a 16ga steel backstop at 90*. Pellets were escaping and the 16ga was getting beat up. I had some 1/8 thick "once fired" scrap that I used for the new backstop at 45* and bent the 16 ga lower box on a HF brake. I used a piece of 1 1/2 strap across the front at a steep angle to retain the pellets. Didn't work, first photo, helped pellets to escape so I replaced it with a 1/4x5/16 bar, top angled toward the inside. 120V Portable MIG did the welding. I cut the steel with a Diablo 7 1/4 metal blade and a framing saw.
With the backstop at 45* the pellets still disintegrate but I am retaining 99%. Still looking for better, whole pellets will reduce oxidation and loss but this will do for now. The target is held with spring clips, which I forgot, so I used a split twig for the session I photographed.