To my understanding, what you need is actually a back stop, pellet trap, target box.... Whatever to sit behind the targets...
I use duct seal pellet traps too. Very quiet when being hit , impressive result in absorbing impacts... The same pellet trap stand well when I upgraded my collection to add a 26fpe .25 magnum springer...
A little bit expensive as I used about 10 bricks to just filled my cookie can pellet trap...
As you shoot both steel BBs and lead pellets, you may need two different target boxes?!
BBs guns usually much less power, what you are using (denim fillers) will be good and very easy to collet the wasted BBs (to re use or recycle)...
For lead pellets, dust seal is good as the reason above, but not very good for BBs as the weak muzzle power on a BB gun could not shoot BBs deep into the duct seal and shouldn't take long, when the BBs hits the others that just sticked on the surface of the duct seal it would easily bounce back to everywhere....
Talking about 'targets', that can be everything... I use paper targets too when I do zeroing or groups. Usually I shoot everything as I am a plinker ~ [emoji38]
Below pls take a look at mine... I happened found a same size cookie can before Christmas time and replaced the one I used for a few years... (use the same duct seal fillers but give it a new home~) in the period of time, maybe 3, 4 years something, I added two bricks to fill the holes when I digged out and removed the lead piles...
Also the 26fpe 25cal power vs pellet tin. My pellet trap have no problem at all for this kind of power. The heavy box with duct seals shakes and wobbles when being hit tho...
https://youtu.be/D8714bSK9js