It is possible for someone to simply throw recycled rubber product on top of an existing steel trap, but that is not how a granular trap is designed to work. A granular trap consists of a metal frame (#10 steel mostly and definitely not AR500 steel) that resembles a staircase, a hopper system...
As good stewards of Liberty and Freedom RTBA folks generally respect other people's property but you cannot account for those that will not assume responsibility for their own actions. For this reason most ranges (even outdoors) I've visited incorporate a liability clause in their range waiver...
Georgia Gun Club in Buford has an Outlaw League twice monthly (draw shoot and move forward of the line/scenario based) and are also featuring a shoothouse (not sure of the dates) for the same.
I want everyone to be trained, especially those that choose to carry, but I would never condone requiring it before someone could arm themselves. That is not Freedom nor Liberty.