Google Search:: "In the April 1994 issue of 'The Airgun Letter,' we provided plans for making
a silent pellet trap. Basically, you stack two courses of bricks of Duct Seal
in a metal-backed wooden frame. Duct Seal is also known as 'thumb gum.' You
may have to check hardware/home supply stores to find it. You'll pay about
$1.75-$2.25 per pound. We bought 12 pounds for our silent pellet trap and
got a nice discount for buying in bulk.

Incidentally, our silent pellet trap design was only recommended for pellet
guns up to the 15 ft-lb level."