Step 5: Choose a cabinet design
Box design can make a $5 driver sound like a speaker that costs $500 retail, but if it's not designed and built correctly, it can also make a $500 driver sound like it was ripped out of an old transistor radio. DIY speaker builders can't make their own drivers very easily, but we do build our own speaker cabinets, so that's where we tinker, innovate, build with care, and shine. As a result, it's the cabinet design and execution that we spend the most time on.
Cabinet design decisions start at the basics, like the volume of the cabinet, whether it will be sealed or ported, how much bracing the cabinet needs, what thickness material it should be made out of and what height the tweeter should be mounted at so that it's in line with the listeners ears.
From there, it progresses to more complex and acoustic decisions like rounding over the corners to reduce interference, building elaborate horn structures to amplify the sound, using exotic materials to further dampen resonant frequencies, line arrays to gain efficiency, mounting drivers at different distances from the listener to accommodate for the fact that high frequencies travel slightly faster than low frequencies, and eliminating parallel faces - the surfaces that create resonant frequencies, by building poly-faceted cabinets, or better, spheres, rather than the standard rectangular cabinet.
That being said, the vast majority of DIY speaker builders start with a straightforward, rectangular cabinet design that even though lacks the bells and whistles, and highly engineered elements listed above, still sounds fantastic.
An example of a MTM bookshelf speaker speaker design appears below from Zalytron.