Treatments consist of bass traps and first reflection points. Bass traps should be placed in corners of the room or corners where walls meet ceiling/floor. First reflection panels go on ceiling and walls where the first reflection point of the speaker relates to your seated position.
Placing foam in that little opening will accomplish nothing, and possibly make matters worse. I meant to say 6" of room behind the speakers to allow the low bass to filter into the room from the rear ports. Again, this is assuming the speakers are up against a wall.
I'm not saying it won't work, you just won't get the best sound for your money. As Rick mentioned, you may want to consider the Architecture W22's and W150 in wall/on wall speakers. Also, Axiom will be coming out with their "T" series in the near future, that will be a flush in wall design.
