I went to home depot and had them cut a piece of 4x8 plywood (actually 2 for mine, as my sink is about 15 feet.) I had them cut the 1 inch plywood first by cutting 2 pieces off the short (4ft) side, 8 inches each. These are used for the end caps of the sink. Then cut 2 pieces of the long side (now 80" long) at 8 inches each. These are used as the sides of the sink. That left me with a 32 inch bottom. I had them cut the end caps to fit, around 33 1/2 inches each from the 4ft pieces (measure to be sure!) and went home and used a 20 guage nail gun and glue to hold it together. I bought a floor drain and cut a hole in one end of the sink for the drain by tracing the inside of the washer for the drain on the sink. I then used fiberglass epoxy (NOT to be used indoors!!) and covered the box on both sides with the goo. it's been fantastic, and strong. I used 2x4's to make a base for it.
Here are some images of the sink...
I hope this gives you some idea.