Since I can't seem to saw a straight cut here is my solution.
Remove the old tube type TV. It was mounted to in a square tube steel structure covered by the surround fascia. Coped in a piece of 5/8 finished plywood that I painted flat black.
Found a flat screen TV that fit well inside the fascia. Mounted the TV to the plywood and remounted the fascia. Made a reasonable shadow box effect. Only drawback is its only a 29 inch TV (if my memory is correct) but it is bigger than the old tube type.
I will try to attach a picture to let you see the effect.
Good Luck
OOPS, inverted picture and can't correct