Pour concrete with a couple of rebars embedded for added strength. The top surface only needs to be level, not trowel smooth. Note that the 2x6 form board was offset from the 2x4 for easy first removal. It's positioned square (90°) with the existing block wall at rear because the RW will follow it, and the RW defines one side of the ground pad. On the other hand, the footprint shape of the drain rock is not important. The 2x4 board will be removed during granite fill to preclude spill out of the drain rock.