Have your builder build the box beams out of rough sawn pine with all the edges 45 so there are no butt joints. Then distress the heck out of the pine beams to make it look like reclaimed wood to match your mantle and barn door. Then you could have the mantle and barn door built out of pine also so all the color and distressing matches. Good way to distress the pine beams is using a wire wheel on a grinder, good luck