For starters

I found two twin size mattresses that someone was tossing out to replace with a new bed, so ...

I took them home and sunned them out for a few days. Then I stacked them against a fence in our yard.

By using some ingenuity with nylon roping I strapped the two mattresses together and made two hooks that are mounted about center in the backstop, and I hang my bag target on them when I want to practice.

I am using a Cabella 23 inch square 10 inch deep hi density bag target with five yellow, red black circles on each side of it.

If you inadvertently miss the bag while trying to shoot from ridiculous distances, then the mattress backstop may prevent a bad accident, like shooting one of your neighbors ... LOL

A King or Queen size mattress can be used also. You can also use some fabric paint to make it look presentable.