When we were building my darkroom a few years ago making a large, shallow, watertight sink seemed the most daunting. While cruising the demo sales looking for doors, cupboards, plywood, and such we found a flatbottomed, 6 ft fiberglass bathtub. Eureka! The bathtub was cut in half horizontally and I have a 6 ft long 7 inch deep sink. The top half of the tub is in the garden with glass on it - our cold frame.