I made this pellet trap a long time ago for shooting my airgun indoors.
Constructed of three bricks of ballistic putty, inside of a vintage wooden fruit crate.
I pulled the small nails holding the crate together and replaced with carpenters glue and screws in order to make it strong and sturdy to contain the bricks of putty.
Just attach paper targets with some small tacks (included) and you are ready to shoot.
$25 cash and carry in Boise. You cannot build this for 25 bucks.