I sloped mine to the back, with a trough on the outside to wash the poop away, put in a mini septic, worked great.
Mine's like this ,except no septic, use the flat shovel for solids.
Colt, could you start another thread about your septic design/
when I poured the slab (sloped to the back) I extended the slab about 20" past the back fence, formed a trough the width of a square nose shovel, sloped it to one end, buried a p trap above the tank, run lateral lines. this was 50yrs ago, were no codes, the ground was rocky so I had good drainage through the lines, I extended the sheet metal roofing out past the trough, this kept most of the rain out.