I would go ahead and build your rebar project. Given the safety precautions already incorporated during range approval, range rules, and personal PPE, the risk of any danger from ricochet is smaller than tiny.
If you want, though this is not necessary, you can make a simple shroud to place over top of your target. (Such a shroud could be made, easily, from three pieces of board or folded sheet metal, and would restrict the deflection of projectiles even further.)