Here is a great design that can easily be copied. http://www.archerairguns.com/trap.html
The back is filled with 1 1/2 inches of Duct Seal (over kill at AP velocities) - a product available at Home Depot. Only use Duct Seal - other sealers and putty products will not work. No noise at all.
I have never had bounce back and only clean the pellets out of the sealer about every thousand shots. Just use a spoon and dig out the glob of pellets and push in a little wad of new Duct Seal as a replacement for what you dug out.
Really on the cheap - I saw one trap made from a five-gallon paint bucked tipped on its side and filled as above.