Working with captainhemo awhile back to design modular 1x12 open baffle H-Frame servo sub cabinets. These are flatpacks that come with all the hardware for stacking to be and Nx12in array. All that is needed is glue and clamps as dowels and bolts are supplied in the kit to build and connect the modules. There are top, middle and bottom module designs to choose what makes sense for your system.
Converting my modular 2x12 H-Frames to 3x12's is as easy as building two more kits. Today did the glue-up for the two 1x12 modules to get them ready to apply the finish and install the GR-Research SW-12-16FR drivers. A 3x12 stack will work with the same servo amp as the 2x12.
Once the finish is fully dried, then just remove the bolts connecting the two modules and put the new middle module in. Once wired in then just install the bolts again to complete the build.
Here are the two 1x12's after glue-up and clamping.