Overall width of the LaScala's is 23.75 inches. The construction is of .75 birch. The inside of the woofer chamber is 15.25 inches. (where the 15 inch woofer is mounted) The inner parallel chamber walls are .75 inches. (1 on each side/total of 1.5) The outer cabinet walls are another .75 inches. (1 on each side/total of 1.5) That's a total of 18.25 inches. (23.75 - 18.25 = 5.5) The remaining 5.5 inches divided by 2 is 2.75 inches. (I hope my math was right!)
I have factory LaScala's, which is what I measured for my drawings. Not being totally satisfied with that, I measured a friend's pair. Same thing. The measurement on both pair was 2 3/4 inches. This seems somewhat odd to me as well but, that's what it is. Just to easy your mind, find a factory pair and measure them.
Tony