Hi jerry,
The chick grit is just a few bucks for a 5 pound bag at the local farm store, I have several more already. As you say, it does flow out easy so I am planning to make a three walled backstop that looks like unblasted stone, to give it something to push back against. I think the technical term is it has a low subsidance angle, same problem they had digging the Panama canal.
Going to be a few weeks before I can get to it, got more RC events and a trip to Maine coming up.