The tile has to attach to the structure of the fireplace. If you wanted support for an arched opening, it probably should have been built before the firebox was installed.

There's not much space; what sizes are the tile and what's the size of the surround?

The tiles are 4.5 inches and that is the space on left and right of firebox. The top area is about 9 inches.

I think your idea is good but only on a conventional wood burning fireplace. We have two gas fireplaces. One in the kitchen and one in master bedroom. We used arched front plates on them.