Lowes has a group of mantle pieces that I got all excited about. It's like Legos for mantles. Choose 1 o f 6 mantles, choose what you like for the uppy part (pilasters can be columns or fluted pieces of wood) put it together and, Voila! a fireplace surround.
In the fine print; 'designed for fireplaces with a maximum of 36" across the fire box'. Ours is 42.125. ..................Back to the drawing board.
So it's a custom length mantle...if you make it wider you also need to make it taller to keep the proportions right.....so that probably means custom.
Roger suggested putting some cardboard up around the firebox and drawing directly onto it. We'll see.
The good news is that the fire box we have comes very close to having the 'magic rectangle' dimensions, so it is only a matter of keeping those proportions and we'll have a beautiful fireplace; someday.