Ruben,
I have another idea for attachment and connection of the modules. No need for extra holes.
Use the existing holes on the bottom and tops and just drill the relief plates to match. This connects the modules together for stability.
IMO, it would be better to use 4 bolts located near the outside edges to keep everything really stiff. Only 2 bolts near the center would leave the 3/4" panels loose and able to resonate and would likely allow some front to back movement/sway on the entire array.
If you plan to use just the 2 holes near the center I would recommend you add at least one bolt in front and back that's centered and about 1" - 1 1/4" in from the edge of the 3/4" panels.
Run the wiring to the back of each box and on the sides to a 12 wide euro style terminal strip at the middle of the stack. This way the modules can be taken apart and moved if needed at a later time. Minimizes wiring runs from the drivers and servo coils to the amps. Also allows for the stack to be reconfigured from 1-6 modules as needed for different trials. Totally modular and reconfigurable. Each module can be shipped independently for weight and size reduction.
My thoughts so far.
That would be a good option for people who needed to reconfigure them regularly.
For people who plan to build and leave them alone, a pre-drilled run would likely be ideal. Nothing else to buy, configure, or install.
Thanks,
Ruben