I used heavy duty liquid nails on the ajoining pieces before screwing them down. One of my EP500 sits on the stage, so I filled the cavities of it with sand, first lining the insides with roofing paper, or a heavy duty plastic would work well.
Keep the A/C 16" apart from your audio/video cable just to be safe. If the cables cross, do it at a perpendicular. I used the vinyl wall cover plates and just drilled a hole in each one for the speaker cable, maintaining one continuous wire.
It's worked out well for me, but I'm sure it's not ideal for everyone. Just how I did mine. Best of luck on it.
