I just stumbled across Hypertufa. I am so excited to try making some stuff out of this. I have been wanting to make a river rock and mortar sand box at the base of my playhouse. But now that I know about Hypertufa, I think I would like the loos of that better. Do you think this would hold up okay? The walls would be 3.5" inches thigh and between 10 to 18 inches high (it will be built into a hill. I was going to reinforce it with concrete mesh. If this is a good idea what would be the best recipe for it. The dimension of the sand bx will be 6' by 6' feet.