This is my solution for an existing square fireplace. I really wanted the linear look but I wanted to avoid completely demoing the fireplace to allow ventilation for a linear burner.
So I decided to frame out a wall in front of the existing stone fp with steel studs and frame a linear niche to give it the linear effect. This also allowed for an extra deep window sill. I maxed out the size of the burner at 36″ to give the flame a linear look. The niche is assembled with a high density epoxy (that doesn’t like to be cut easily :)
For the face of the fireplace, I drywalled and installed expansion joints to give it an added linear detail. After some smooth coating, I applied a grey venetian plaster with a combination of trowling and toweling to achieve the look I wanted.
Finally I poured in lava rock and topped it with fireglass, which radiates heat much better than wood or firelogs.