Mike Dodd wrote:

Hmmm, pondering how you'd do that.... presumably a jig to fit on top
of the draining area, and sand one edge slightly to create the
gradient?

I saw something about that in a woodworking magazine recently. It is a jig
which tapers so the router cuts deeper as it moves along - but not by much.

Probably take longer to make the jig than do the job.

Dave