What do you have to work with? What is the lay of your land? Do you have any ravines or hills that you can shoot into? Do you have any dirt moving capabilities? Do you have any railroad ties at your disposal? And most important, do you have any close neighbors? Have a heavy duty truck? I have a steel backstop for sale. 17'w x 12'h
My 3 acres is perfectly flat. I put three old telephone pole sections on the ground and then put boards across the back side of these and angled brace poles to the front and two dump truck loads of dirt behind then a backhoe come in to pile it up well enough. It seems to work well enough so far.
I've never seen a high powered expanding rifle bullet go more than a couple of feet into dirt, but if you have at least three or more feet of dirt it should stop them. If you're firing a .22 a foot of dirt will stop it.
The telephone pole sections are about 10 feet long with three feet in the ground. There are three of them in a line. The poles to brace these are angled to the front (where the shooter would be) and spiked into the telephone poles and planted in the ground. These poles are about eight feet long with about two feet in the dirt. One for each of the telephone pole sections. The back is covered with scraps of treated 2 by of various widths and some old creosote fence posts. These are on the back of the telephone pole sections so the dirt will push them against the poles. They are spiked into the poles. The dirt is piled behind that. The whole backstop is about nine feet tall and about 12 feet wide, but since the dirt is a mound, I wouldn't consider it all safe to shoot at. It's there to hold the dirt. But I'd only use about half it for a target area.