Thanks for the comments,
In the BB trap shown, note the rods that go from side to side. The lower one, the front one, is used to clip any target to it. And folded newspaper pages are hung over/clipped to the back rod, the higher one, to keep most all of the BB's in the trap/box.
Here's a link to how to make your own -
http://www.pointshooting.com/airsoft.htm You can add a pie tin (gong) to it to let you know where your hits are going, and you can set it up so it swings from side to side if you have the room for that.