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Here is another picture with the black tile extension -

how wide is a 72-in TV? (that's the diagonal measurement)

you definitely don't want the TV to look wider than the firebox, so some black tile might be the solution, although I'm not sure I'd go quite as wide as in your last photo...

OP, I see where you are going with this concept. I think it actually isn't a bad idea to visually expand the firebox with the black tile on either side. Please note, I'm not a designer at all, just a consumer like you.
I see acm's point that there is a proportion that works and without knowing the actual TV width, as opposed to the diagonal measurement, it's tough to know if the 72 inch TV works on that specific fire box.
Hopefully one or more posters can give you additional pointers.
Have you considered having more black tile on the FP so that the TV completely disappears into the black background?

@Zankar Thakkar - exact same isuue! what did you finally do? Can you please share pics?