Plumbing has been redone for improved efficiency and to easily connect standard garden hose. Also fabricated a poly-carbonate case (3/8" thickness) to protect from weather and provide a mount for the hose connections. See updated pictures.

Also, one small modification to the control board: I had to add a 1M-ohm resistor across C101 to discharge more timely (~1s). I had relied on leakage currents in the FETs to take care of this but it took hours.