Not that it addresses your question, but why care if it's in the sump? Just slap the thing on there any way that it will fit and close the door. Under my cabinet is a disaster area but everything does it's job even if it is wedged in sideways tacked up with chewing gum and duct tape.
On topic, a couple of boards going counter direction and lay it on there...
If ascetics matter.. drill a couple holes in the top of the fixture and hang it from the top..
All assuming that your ref is under cabinet of course.
All proffered in a IMO, non offensive, nicey nice posting..............IMO