The next step is to place the power, BeagleBone Black, and the speakers onto the base plate. Things start to take shape in this step.
First, carefully separate the cape from the BBB. This takes a lot of effort to do, and care must be taken to not damage the boards or the pins as this is done. Use 4 M3 screws to attach 4 M3 standoffs to the bottom of the BBB.
Prepare the speakers for mounting by attaching 4 M4 standoffs with M4 screws to the bottom of each speaker like so. I added rubber washers between the speaker and the standoffs, hoping that this would prevent any strange vibrations. I'm not sure if it does the least bit of good.
Leave these screws somewhat loose at this point, as they will need to slide during installation.
Now position the BBB over the base plate. It goes in the very center of the large piece of acrylic, and is secured with 4 M3 screws. The ethernet jack should point towards the player 1 side. The standoffs are not symmetrical, so if things don't seem to line up, try flipping things around until they do.
Secure the speakers to the acrylic with 4 M4 screws each. I am not sure that the holes are positioned correctly in the cutting file. It took more 'fiddling' the make everything line up than I would have expected. By starting out with all of the screws very loose and slowly tightening them down, things seemed to go in place, and the speaker cones are not visibly bent. This is something to check before cutting acrylic.
There are holes in the base plate for the audio amplifier, but I haven't found standoffs that fit the holes in the amplifier board. It can just rest in place, as the wires are stiff enough to prevent it from moving around too much.
The last step for now is to attach the power bus. I have pictured the barrier block that I used below. There are two places on the base plate where a barrier block can be mounted. The outer four holes will be used by a barrier block. Your barrier block may be different.
The inner two holes of the power mounting area are for the white Power Bus blocks. I beleive that they fit, but have not used them in my assembly.
To attach either kind of block, use M4 screws, and M4 nuts. The holes are cut to this size in the base plate.
Once the blocks have been mounted, power can be run to the BBB, and the wires from the display panel can be attached.