Have you tried use Kitchen equipment stores, they usually have tons of used stainless sinks, trays and large venting systems that go for excellent prices.

I can email you a jpeg of our venting system if you want , it is basically a very large tube running the whole length of the darkroom , in a L shape, the only holes are at the bottom of the tube , and this is then hooked up to a very powerful exhaust fan.
works well for our enviournment , specifically when I am sepia toning and the whole room smells like the back end of a cow.

I think any competent construction worker could put this together.