Exactly my plan!
Just crazy what non shooters were suggesting at work because the material is free.
OK SO..., there are several ranges around where I live, and ALL of them do what the others have suggested, used railroad ties OR tires filled with dirt, or a semi-walled "pistol butt" area, BUT they then cover the ties or tires, with sand and dirt, or the pistol butt is filled with sand and vermiculite. The police have a "tire house" range that has walls made of used tires, but that's way more than most ranges need or have. These are for the bullet impacts, not for posting targets...
At these ranges, they use several methods for target holders.
You can use U-Channel posts, set into the ground,, and place a piece of plywood between them, which makes it easy to swap out a new piece of plywood when the old piece is shot to bits..., which also keeps the target holders in a fixed position....
The same thing can be done with variable range settings, by using PVC set into the ground, so the shooter may pull up the target holder, move it to a different fixed position, and insert it into the PVC pipes there, BUT can't get into trouble putting them at odd angles. The last method is with "feet" which means the shooters can move these to any distance, but..., they might put them in locations pushing the "safe" boundaries of the impact zones.
They can be made of wood, but the U-Channel steel holds up pretty well. I just use wire supports from old campaign signs to hold my targets when I need to shoot at a distance that the fixed holders won't give me. That's an OK option, but... if you get strong winds the targets will flex back and forth with the winds and IF you hit the wire with a bullet, well, time to get a new wire support, while the U-Channel stuff, although needs a guy with welding skills to make, tend to hold up with being shot
LD