I'm looking for some kind of gizmo I can mount on the inside of my gunwales,
that is flush with the plane of the gunwale, that I can use to tie off my
fenders to.

I've been considering those gadgets commonly available in boating circles
known as flush hatch or deck pulls. I know they require 4 small holes for
mounting screws. But, is there also a need to route out or drill a larger
hole for the body of the device to recess in? Or, do they just screw onto
the bulkead and lay flat?