I built a large darkroom sink out of common materials and used it for many years until I sold it. I needed something deep, and big enough to take four 16x20 trays. Basically I used 8"x1" pine for the sides and ends, screwed and glued together and with a metal "L" bracked across each corner. The base was simple masonite, supported on several cross-pieces and an underneath bede around the perimeter. I let a drain outlet into one end (where I glued an extra pad of masonite underneath for extra thickness). The secret of the whole thing was that ordinary materials could be used, because all of the wet surfaces were coated in several coats of polyester resin - the type that's used for setting fibreglass mat. I had no trouble with the sink in the 15 or so years I had it. This sort of thing is very do-able. Good Luck.

Barry