Placing a speaker in an enclosed area is not really the ideal, but in the real world most of us are forced to make compromises when it comes to placement.

If you can place the M3s so that the front of the speaker is a half an inch to an inch, or so, out past the edge of the shelf. That will help minimize reflections.

If you can get 3", or so, of space behind the speakers, that should give the rear port enough room to "breathe."

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton