Simon, here are the steps I would use to make a hole-cutting component from your sinks.
Select all of the geometry.
G to create component.
Set the Glue to: direction.
Tick the box for Cut Opening.
It’ll look like this at that point.
Click Set Component Axes.
Place the component’s origin where you want it along the bottom of the rim of the sink.
I chose the midpoint along the front edge.
Set the direction of the axes. In this case I chose Red along the front and green toward the back. ( I added a guideline only to make the negative green axis more visible for the screen shots.)
After setting the green axis, the Create Component panel will reappear. Hit Enter or click Create.
Sink easily drops into place on a horizontal face and cuts the opening as expected.
