I have some in-wall speakers that I've installed in speaker backer boxes. The reason I used these MDF backer boxes is because the cavity on the right side of the diagram is actually a built-in furniture face with a rackmount enclosure at the bottom and an HVAC vent coming out the top portion. By using the backer boxes, I've given the left and right speakers roughly the same acoustical enclosure properties.
I've contacted the manufacturer (HTD). They won't give me the spec sheets for the drivers, but they state that the drivers are meant to be used in an infinite baffle enclosure (i.e. a stud cavity) and should have an enclosure volume of at least 1.5 ft^3. Unfortunately, my MDF backer boxes are 1.15 ft^3 at best. I can increase the perceived volume of the enclosure by adding damping material (fiberglass).
I can also drill holes up through the top of the enclosure and though the top plate into the attic space above (likely, two 1" holes). Will these holes help my enclosure act more like an infinite baffle design or will it act as a port directing sound into the attic?