In reply to CrowdLeader:
Not sure about formal research, but I have seen heavy holds hit rubber crumb and bounce off a fair distance.
My preferred option is fine scaffold netting over Corden netting. But either work to stop the hold.
I would say a cone and ropes don't really cut it.
I would say a good few meters around the work area and from anchor point as a minimum fenced off.
Generally customers will come stand close at the edge as they are interested in the process. So prefer to set when closed off or quiet.