I'm considering adding a tiered flagstone spillway to my spa by myself. Is the spillway just pieces of flagstone stacked on top of each other? I know the stacked stone is in the front of the spa but what about the back inside the jacuzzi? Is that plaster or stone also? Questioning how the spa holds in the water when it's raised with just the stone and no plaster. Been looking online for pictures but can't find any that are inside the spa. Any help would be greatly appreciated.