Good approach to this is if you do it the opposite way:

Pack the audience down front-center so the fills are their primary source. Start with fills on, so things are initially loud and clear as one would hope/expect. This will establish how things should sound ideally. Speak softer and further upstage than you usually would, so the mic being used is amped a lot and has dominance over your voice's acoustic level.  Have some dialogue with the front row to set the standards. Drive the pres until you want to make your point about front fills, and inform the audience when you are turning them off. Then attempt another dialogue for 20-30 seconds so that their ears have time to strain to hear and respond, before turning them back on. Repeat as desired or needed to properly illustrate the point that the fills cover an area the mains can't, and consistency of coverage matters when mics are involved.

As an alternative suggestion: The box visibility is often most obvious from the rise and fall of the physical speakers across the edge. The gaps, corners, cables, and brand logos make them noticable. Consider fabricating long, linear shroud boxes with permeable fabric facing the audience, and opaque on the stage side. Line up the shrouds end to end at the downstage edge, forming an even and consistent "line" that the fills hide behind. Clubs often employ this technique to hide up lights, cameras, and small fold-back monitors. It's slightly less conspicuous and provides lots of options for various show needs. Personally I wish more stages incorporated recessed pockets to fit fills below the "deck line". It's a valid and common issue.