I've had success in the past with fabrication of a piece of sheet metal to fit the outside dimensions, then cutting the appropriate opening in the middle for the new unit. A little spray on paint, and it looks not half bad. If the heater rating will allow, you can even use 1/4 inch paneling or plywood. There is also a wedge-on piece you can add to the edge of sheetrock to finish the edge, then paint, and it blends into the wall, even if it sticks out an additional 1/2 inch. You can also frame it in with mitered trim, like a picture frame.
