i flushmounted midfields and nearfields some times ago. I`d do this when the room needs flushmounting speaker, most in evil small rooms, in case of no place for proper walldistance.
I useed 2 layers of 38mm HDF and 12,5mm drywall on top with good results the last years.
Most of the time the mounted speakers had a low cutof because of the use of a sub or a sub array. so i have no realy experience in flushmounting a fullband used speaker.
I`m also not shure about the rear ports of the mackies, since the bassreflex port is part of the speakers freq response.
But from my personal experience i love also flushmounted nearfields.
maybee barefoot is a great person for you, to ask for your idea. he`s a friendly and helpfull person who loves to share his knowledge.
cheers
constantin