Ours is just a cabinet we had the cabinet shop include when we did the downstairs kitchen and bathroom. I believe the inside width was 19 3/4", and we just framed in an opening in the wall so that the cabinet could be mounted just inside it. It sits up off the floor about 7" as shown, with just some framing the cabinet rests on. The cabinet shop pre-drilled all of the holes for the adjustable shelves (all 3/4" MDF).
In my case, the rear opens into a storage room so the back is totally open. This is good since we put the glass door on the front, otherwise the equipment would get too hot. If you are going to enclose the back, you may want to leave the front open. If you want to close the front with glass, you will need a fan at the top to pull the warm air out, along with a intake vent at the bottom.
I do very much like the glass door as both my DVR and AVR make noise. The front door silences it pretty well.
Let me know if I can be of any further help.
