Excuse the poor perspective photoshop edit, but in the future I would love the option to craft/spawn only the insides of a regular furnace. Also the ability to craft a coal-chute to place within a limited distance of the furnace insides. I made a few concepts as to what this could look like:
As for the first concept(the bricks) realistically you would not be able to place the furnaces so close due to the depth of both their insides colliding with each other.