My current darkroom sink I built five years ago. It started as a simple 1x12 box with a 2x2 frame inside to hold the sink bed. The frame was designed to provide drainage to the middle, but one could slope it to a back corner as well. The bed is half-inch marine plywood. The corners between bed and wall were fared with marine epoxy. The whole thing was primed and painted with marine paint. I'm more than happy with it.
Here's a picture of it during installation (minus faring, drain and final topcoats of paint):
View attachment 256726
It sits on 2x4s lagged to the wall studs and on the supports used to make under-sink storage.
I've got plans somewhere still in pdf form. Just PM me if you're interested; I'd be happy to share them.
Best,
Doremus