We build a "Masons Frame" for installation in new construction. It is installed in front of the firebox prior to the masonry veneer. The frame is built heavy so that the mason can stack his rockwork over the structure.
The fireplace doors are attached with bolts to the masons frame so the door is truly "built in" to the wall. This system allows for ease of installation and can be shipped anywhere.

overhead view of the mason's frame