I've seen a flannel sheet hanging in my friend's garage catch field tips neat as can be without tearing. He hasn't tried it with broad heads. Medieval Japanese horsemen would wear a piece of fabric fastened to the back of their armor and it would fill up like a sail when they rode. It would deflect arrows shot at them while they rode away. Based on that, a simple fabric backdrop might work just as well on stone points. It must hang loose from the top corners, not be stretched tight, and not fastened at the bottom. I don't know if your points would break against the fabric or not. You would need a soft floor for them to land on, after bouncing out of the sheet, to keep your points from breaking. A piece of tape or a silhouette inked onto the sheet might work for a target.