Ok, here is what I got;
First off.. these do not appear to be precision made

The measurements from one side to the other differ by varying amounts.. from 1/16" to about 1/4". All wood used is 1/2" thick with the front piece rounded over on the edges. I believe the 'target' size was 7" wide by 8" high and 3-1/2" deep. The cutout for the panel is 3-3/4" wide by 5-1/2" high. The bottom and side panels are 3" wide, which attach to the rear of the front panel giving the extra 1/2" depth. The tops of the side panels are angled downwards toward the rear by about 10 degrees.. the front measurement (where it attaches to the front panel) is 8", the rear measurement is 7-1/4". There is no top piece, and the hole in the bottom where the wires enter is 1-1/8". I've attached a couple of pictures that may be of use as well.
I'll have to defer to others as to how it was attached.. mine came to me in a box of other stuff the PO had removed for repair/refinishing (photo below). I'm still trying to figure out where half the stuff went
Cheers,
Brad