I use duct seal in a home-made wooden frame... I print my own targets on letter-size (8.5" x 11") paper and the opening size of my wooden box is 9" x 11.5". It took 8.5 "bricks" of that duct seal (at $2.99 each) to fill, and I made a friction-fit frame that fits into the opening of the box to hold the paper target down in place. I shoot in my back warehouse after work and the area behind where I place the target is perfectly safe even if I miss the target completely. But at 10 and 20 meters, I'm not missing it.
Here's what it looks like:
I bought my duct seal at Home Depot and it's a dark grey material. Worked perfectly. So well in fact that I made a second box so that I could set up two of these so that two shooters could shoot side-by-side at the same time. Only problem was that I'd cleaned out the entire stock at the local Home Depot for my first box. So I went to Reno Depot and bought a "similar" duct seal there. It was $3.19 per brick (about the same size). It was much lighter in color. Anyway, the material was not the same and it was WAY too sticky!!!! So sticky in fact that my paper targets would get completely stuck to the duct seal and would be difficult to remove without tearing and without making a mess on the back of the target. And the stuff was much lighter in weight (meaning less dense?) as well.
So being unhappy with the second box, I drove out of my way to ANOTHER Home Depot and bought some of the "good stuff". Replaced the crappy Reno Depot stuff and now I've got two of my target pellet traps that work perfectly.
Even with an 800 fps rifle at 10 and 20 meters distance, the pellet doesn't fully embed in the duct seal and is very easy to flick out (which is what I do after shooting). The sound of the pellet hitting the duct seal is VERY quiet and there are no ricochets. I'm really pleased with this.