Best homemade backstops....

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I've thought about this quite a bit. A double/triple thickness depth of stacked, used up car/truck tires, filled with gravel or sand should work great. The tires wont shatter like wood, just absorb and then go back to natural shape. They are cheap in most places, too. Easy to make up and define the area you want to use, stack neatly without falling down, etc.

Yep, fill with dirt or sand and call it a day.