Totally agree with JohnKSa.
Monkey Sh.........err Electrical Duct Seal has made for the best most durable targets I have ever used by far. As an aside if you go looking for it in a hardware store they may not know where the electrical duct seal is but if you ask for Monkey Shi...... they will probably go "oohhh that stuff."
Here are my two current traps.
This one was my first one made about 10-12 years ago. Used a very small steel junction box and absolutely packed it tight with duct seal. It will stop a .22 LR. Bolted a handle on top, some binder clips on the front and some rubber feet on the bottom. Has been doing its job with aplomb for a decade or more.
This second one is about 2 years old and built because I wanted to use 8.5x11 sized targets. Again I bolted a handle on top and binder clips on the front. No feet on the bottom as it sits flat. Still a steel junction box but with a thinner layer of duct seal as I realized how efficient this stuff was at stopping a pellet. Again going on 2.5 years of doing its job with no muss no fuss.
Both probably cost about $50 bucks to build maybe less.
Cheapest all steel junction box at the hardware store
Some binder clips
Good solid handle from the hardware store
Maybe some rubber feet
Some misc. bolts/screws
10 or so bricks of duct seal
Just gut the box, drill/screw your hardware into place. Pack in the duct seal spreading it all through the box as deep as you need. It stays pliable. I have never had to replace the seal. I have only cleaned pellets out now and a great while.
Hope this helps.
Chris