Actually, this is basically a self-draining planter box with fill (albeit big and with different materials). All you need to do is set your fencing evenly around the rough dimensions of 9 x 48. But, instead of carrying the fence line down the slope, you just make the fence line straight and even down to the ground. With the box made, you just fill inside of it to be level with the aggregate materials. Hard part will be to properly put the perforated pipe and filter fabric in the box.