For something like that you would have to build an overhead compartment across the whole shell so everything sits flush. You could even run the speakers in there.
Here was my plan:
A monitor/player like this
mounted so when you fold it down it faces the back of the cab.
full range speakers mounted at the corners and sub sitting in the cab with the rear window open.
If you want a bigger screen you could use this 14.1 inch monitor
but you would have to send video to it. Maybe from a laptop sitting in the cab