I'm still futzing with the sink but I think I'm about 99% there. I started with a block of wood, a bowl from Home Goods, a pump from Bacons junk bin, and a chunk of roofing rubber from Ace Hardware. I also got a scupper from West Marine. The rubber will ride up on the bulkhead separating the main cabin from the head. I'll use a flexable tube for the drain going into the head. The sink water supply will be from a collapsible plastic 5 gallon jug.
Sink in the up position
With rubber scupper, I'll cut the excess rubber at some point.