Some of the back boxes I have seen are so lacking in cubic volume that I have to ask why? Think of a sub/satellite system, where the sats are really speakers in boxes that are too small to extend the LF range. You know what kinda bass they put out - none, which is why a sub is required. If there is not enough volume inside the enclosure, the resonant frequency goes up, up, up, to where there will be no improvement in the bass, no low end at all. In that case, an infinite baffle is way better, and is not really that bad.

For experimental purposes, I would start simple. Cut some 2 x 4 or 2 x 6 at 14–1/2 inches and just frame in the cavity to get the proper cubic volume. Caulk all the seams to prevent leakage. You should get a pretty good approximation of whether it is going to be worth your trouble to start building back boxes.

I would agree with other posters that most rooms are background listening and not critical listening rooms, so you will be putting in a lot of effort, without a lot of return.

For those rooms where are more critical listening is desired, it will be a big jump upwards in trouble and in the budget as well.