I was shooting in my 30x40 foot shop last winter. End to end gave me a perfect 10 yard range. Guns ranged from a Crosman 1377 to an RWS 350 .22. My pellet trap is basically a box of 1 1/8" form board plywood. The top is cut at an angle, (picture a cube cut in half from corner to corner) and I screwed on a piece of 3/8" thick aluminum that I had on hand. I cut the angle at 45 degrees, if I were to do it again, I'd make it more box and less triangle, with the back at maybe 80 degrees. Although my current design has held up fine to thousands of pellets, I think a straighter back would flatten and drop the pellets a little better. Size is approx. 2x2x2, with a with a 2x4 on edge on the bottom as a lip to hold everything inside. That last piece just got modified, I drilled some holes in the top to hold golf tees, add paintballs and you have a great alternative to paper punching :-). You can either leave it open, or stuff it with rags, etc, to help soak up pellets. At that range, I only missed the target once, and that was a lapse of judgment on my part: had my finger on the trigger before I was ready to shoot. Fortunately, it was a low powered gun, and I just dented a roll up door, instead of putting a hole in it. I'd still suggest a solid backstop, maybe a sheet of 1 1/8" plywood behind the pellet trap. If you'd like more info, I'll try to take some pix later. Later.
Dave