I use empty 12 pack beer bottle boxes filled with rubber mulch, shooting my .25 Mrod at 50FPE at a point blank range without any issues.When I need a new box it is a good excuse to drink some more beer.. Cheers
I use corrugated cardboard I get at my workplace. A bundle of 30 or so 2'x2' sheets is placed in this open front wood crate. When one side gets shot up I turn it around, when it gets to ragged I separate the pieces and collect most of the pellets for recycling. A bundle like this is good for several hundred shots. I tape my targets to individual pieces of stiff corrugated, wedge them against the backstop with a brick at the bottom edge.
Those realty signs that push into the ground work extremely well for lead pellets in guns up to about 25 FPE. They dent up pretty good but will last quite a while before you need to replace the steel sheet. The sign absorbs the energy extremely well so pellets don't bounce back with any force, they just hit and fall within a few feet from the sign. I have one I found in the woods, use it for my pumpers and CO2 guns. If you have a gun that punches through it you can use slightly thicker sheet steel. Alloy pellets, round balls and BBs can ricochet or punch through so I don't recommend it for those types of ammo.